Internal Medicine

Internal medicine concerns adult health and disease across organ systems. An internist may investigate a new symptom, manage several chronic conditions together, or care for a severely ill patient in hospital. The American College of Physicians describes that broad scope. The chapters below connect normal physiology with the findings that prompt investigation.

This is a short introductory textbook for interested readers and beginning learners. It explains clinical reasoning and selected conditions; it is not a complete clinical reference or a guide to prescribing. The cases are fictional teaching exercises. The text was produced with AI assistance and has not undergone independent clinical review. Use qualified clinical care for personal medical decisions.

What changed?

Start with the patient's account. A symptom is an experience: breathlessness, pain, nausea, fatigue. A sign is an observed finding: a raised temperature, swelling, an abnormal heart rhythm. Neither category is inherently more important. A person can report a serious symptom before an examination reveals its cause.

The time course often narrows the problem. Breathlessness that began suddenly during a walk raises different possibilities from breathlessness that has progressed over six months. Ask when the person was last at their usual level of function, what happened first, what followed, and whether the course has been continuous, episodic, or worsening.

The word dizzy is particularly unhelpful until clarified. It may refer to spinning, near-fainting, imbalance, or an ill-defined lightheadedness. Each description points toward a different set of mechanisms. Replacing the patient's word too quickly with a diagnostic label can send the rest of the assessment in the wrong direction.

A problem representation

A clinician condenses the history into a short statement that preserves the features most relevant to diagnosis. Consider this fictional example: an adult with two weeks of worsening breathlessness on exertion, new difficulty lying flat, and swollen ankles. That representation retains duration, functional effect, positional symptoms, and fluid-related findings.

It suggests a cardiopulmonary problem with possible congestion. It does not yet establish heart failure. Lung disease, anemia, kidney disease, medication effects, and other conditions can contribute. The next observations should help distinguish those possibilities rather than simply confirm the first attractive explanation.

A differential diagnosis is an organized set of explanations. It should include likely causes, dangerous causes that require timely exclusion, and alternatives suggested by specific findings. A list of every disease ever associated with breathlessness is less useful than a short list with reasons for and against each candidate.

Assessment and urgency happen together

Severe breathing difficulty, new chest pressure with concerning symptoms, sudden neurological deficits, uncontrolled bleeding, collapse, or marked alteration of consciousness require urgent assessment. In a real emergency, contact the local emergency service. A complete interview should not delay help for an unstable person.

Once immediate needs are addressed, examination and testing refine the explanation. Vital signs describe part of the current state; trends can reveal deterioration that one measurement misses. Medication history, allergies and their actual reactions, recent procedures, travel, exposures, family history, and the person's baseline function can all change the interpretation.

The examination requires explanation, consent, privacy, and attention to what the person can tolerate. Communication support matters when language, hearing, cognition, or distress interferes with the account. A family member's description of an acute change can be valuable, while the patient's own perspective remains central whenever it can be obtained.

Leave a reasoned record

Separate observations from conclusions. “Breathless after walking across the room” records a functional limitation. “Probably cardiac” is an interpretation. A good note allows another clinician to see how the interpretation arose and what remains uncertain.

Write what would change the plan. A pending test needs someone responsible for checking it. A person going home needs a follow-up arrangement and clear circumstances for returning sooner. AHRQ's work on diagnostic safety during care transitions addresses how information and responsibility can be lost between teams. The record needs to carry these responsibilities beyond the consultation.

A test changes the probability of a diagnosis. Its result acquires meaning from the question being asked, the person tested, the method used, and the time the sample was obtained. A number outside a laboratory interval needs interpretation.

MedlinePlus explains reference ranges and why laboratories may use different ones. Some healthy people have results outside an interval, and some people with disease have results inside it. Units and the laboratory's own interval belong beside the result whenever it is copied into a note.

Sensitivity and specificity

Sensitivity is the proportion of people with the condition who test positive. Specificity is the proportion without it who test negative. These definitions start with disease status. A patient's question after a positive result runs in the opposite direction: among those who tested positive, how many have the condition?

That quantity, the positive predictive value, depends on how common the condition was in the tested group as well as the test's performance.

Here is a wholly invented test with 90 percent sensitivity and 95 percent specificity. Apply it to 1,000 people, of whom 20 have the condition.

There are 67 positive results. Eighteen are true positives, so the positive predictive value is 18 divided by 67, approximately 27 percent. The test correctly identifies most affected people, yet most positive results in this low-prevalence group are false positives.

Now apply the same invented test to 1,000 people, of whom 200 have the condition.

The positive predictive value is now 180 divided by 220, approximately 82 percent. Sensitivity and specificity were held constant in the exercise. The starting population changed.

These are arithmetic examples, not performance claims for any real medical test. In practice, performance can itself vary with patient characteristics, disease stage, thresholds, and the setting in which a test is used.

Choosing the next observation

A useful test separates explanations that would lead to different decisions. Ordering another test merely because it exists can add ambiguity, especially when the starting likelihood of the target condition is very low.

Suppose an unexpected result conflicts with the history and examination. The clinician considers several possibilities: the working diagnosis is wrong, the test has limitations, the specimen or measurement was affected, or more than one condition is present. Repeating or confirming the result may be appropriate, but urgent findings cannot simply be dismissed as error because they are inconvenient.

Time matters too. A marker may not yet have risen early in an illness. A negative result at one time point can mean something different from a negative result obtained later or through a validated sequence of measurements.

More tests create more flags

A second invented exercise illustrates a statistical cost of broad testing. Assume 20 independent measurements, each with a 95 percent chance of falling within its reference interval in a healthy person. The probability that all 20 fall inside is 0.95 multiplied by itself 20 times, about 36 percent. The probability of at least one outside result is therefore about 64 percent.

Real laboratory measurements are not all independent, and reference intervals are not identical to diagnostic cutoffs. The example explains a general problem: increasing the number of observations increases the opportunity for an isolated surprising value. It does not justify ignoring an abnormal result.

Read the result in context

Before interpreting a report, identify the specimen, units, method-dependent range, previous results, and relevant treatment or preparation. Ask what conclusion a positive or negative result could support and what it could not establish alone.

The same discipline applies to images. An abnormality found on a scan may explain the symptoms, contribute to them, or be incidental. The next step depends on the relationship between the finding and the clinical problem, including the consequences of investigating or leaving it alone.

Air reaches the alveoli through conducting airways. Across the thin alveolar-capillary interface, oxygen enters the blood and carbon dioxide leaves it. Ventilation moves air; perfusion supplies blood; diffusion moves gases across the interface. Effective gas exchange requires these processes to work together. NHLBI's account of breathing describes the basic arrangement.

Breathlessness can arise from failure at several points: narrowed airways, impaired lung tissue, fluid in or around the lungs, abnormal pulmonary blood flow, weak respiratory muscles, reduced oxygen-carrying capacity, or increased metabolic demand. The sensation alone does not identify the location.

Oxygen saturation is one measurement

A pulse oximeter estimates the proportion of hemoglobin carrying oxygen. It does not directly measure the amount of hemoglobin, the total oxygen content of blood, or the adequacy of carbon dioxide removal.

Consider a fictional patient with anemia and an oxygen saturation of 98 percent. Most available hemoglobin may be oxygenated while the total amount of hemoglobin is low. The total oxygen-carrying capacity can therefore be reduced. Similarly, a patient can retain carbon dioxide despite an acceptable oxygen saturation, especially while receiving supplemental oxygen. The British Thoracic Society oxygen guideline explicitly distinguishes saturation from oxygen content and carbon dioxide assessment.

The FDA describes pulse-oximeter limitations, including factors that can affect accuracy and concerns related to skin pigmentation. Clinicians interpret the reading alongside the person's symptoms, examination, and other measurements. Severe breathing difficulty warrants urgent care regardless of a single home-device reading.

Airflow obstruction

Asthma involves variable respiratory symptoms and variable expiratory airflow limitation. Spirometry measures how much air a person can exhale and how quickly. Testing before and after a bronchodilator can demonstrate a change in airflow; other tests may be needed when the diagnosis remains uncertain. NHLBI's asthma diagnostic guide explains these approaches.

A normal examination between episodes does not settle the question. The history of triggers, nighttime symptoms, episodic wheeze, cough, and response to previous treatment helps determine the next investigation. Wheeze itself can have several causes.

COPD is associated with persistent airflow obstruction in a relevant clinical setting. Exposure history includes tobacco smoke and other inhaled hazards. Spirometry is central to its diagnosis; cough or a smoking history alone is insufficient. Asthma and COPD can overlap in features, and a bronchodilator response is not an absolute boundary between them.

Treatment of obstructive disease combines the appropriate inhaled medicines, correct device technique, exposure reduction, and a plan for worsening symptoms. The particular regimen depends on the diagnosis and current guidance. An inhaler can contain an appropriate drug and still deliver little benefit if the person cannot use the device effectively.

Blood flow can be the problem

Pulmonary embolism occurs when material, usually a clot arising in a deep vein, obstructs pulmonary arterial flow. Symptoms can include sudden breathlessness, pain on breathing, a rapid pulse, or fainting, although presentations vary. NHLBI describes the condition and diagnostic tests.

D-dimer is a marker related to clot breakdown. A raised result is not specific for pulmonary embolism. A negative result can help exclude the diagnosis in selected patients within a validated clinical pathway. NICE's venous-thromboembolism guideline combines clinical probability and testing rather than treating D-dimer as a universal yes-or-no answer.

A fictional comparison

Two adults report breathlessness on exertion. One has episodic wheeze with variable airflow limitation. The other has progressive fatigue, a low hemoglobin concentration, and no evidence of airflow obstruction. Both symptoms are real; the mechanisms and treatment priorities differ.

For each, ask where oxygen delivery is being limited. Is air moving, are gases exchanging, is blood flowing, and is enough functioning hemoglobin available? This physiological sequence helps organize a differential without assuming every breathless patient has a primary lung disorder.

The heart must receive its own blood supply, fill adequately, eject blood, and maintain a coordinated rhythm. Problems in any of these functions can cause overlapping symptoms. Chest discomfort, fatigue, breathlessness, palpitations, and swelling each require interpretation in context.

Ischemia and injury

Myocardial ischemia means the heart muscle's oxygen supply is insufficient for its needs. An acute coronary obstruction can cause infarction, but ischemia and myocardial injury also occur in other clinical circumstances.

New chest pressure or discomfort with breathlessness, sweating, nausea, collapse, or other concerning features requires emergency assessment. Heart attacks do not always produce a dramatic central pain. NHLBI's symptom guidance emphasizes seeking emergency care when an attack is suspected.

An ECG records electrical activity. Troponin testing detects myocardial injury. Clinicians interpret these with the history, examination, timing, and serial measurements when indicated. The diagnostic account describes their roles. An early nondiagnostic ECG or troponin result does not universally exclude an evolving acute coronary syndrome.

The universal definition of myocardial infarction distinguishes myocardial injury from infarction. Troponin elevation alone does not tell the clinician that an acute coronary clot caused it; evidence of ischemia is part of the diagnosis.

Heart failure is a syndrome

Heart failure involves symptoms and signs arising from structural or functional cardiac abnormality. It can produce congestion, impaired forward flow, or both. Echocardiography helps assess ventricular function, valves, and other structural features; laboratory and additional imaging findings contribute to the assessment.

Ejection fraction is the proportion of the left ventricle's filled volume expelled with a beat. If an illustrative ventricle contains 120 milliliters before contraction and ejects 70, the fraction is about 58 percent. That is not a statement that 58 percent of the heart is working.

Heart failure can occur with reduced or preserved ejection fraction. The 2022 AHA/ACC/HFSA guideline emphasizes evidence of increased filling pressures when ejection fraction is above 40 percent. A preserved fraction neither establishes heart failure nor excludes it.

A ventricle that fills abnormally can require high pressures to receive blood even while ejecting a substantial fraction of what it contains. Those pressures help explain congestion and breathlessness in some patients with preserved ejection fraction.

Treat the mechanism and the course

Relieving congestion and improving long-term outcomes are related but distinct treatment goals. Diuretics can reduce excess fluid. Other heart-failure medicines act on pathways involved in disease progression, with the evidence and choice depending on the type of heart failure and the patient. The professional guideline includes several complementary medication classes for reduced-ejection-fraction disease.

The plan also addresses causes and contributors: ischemia, valve disease, blood pressure, rhythm, kidney function, and other conditions. Monitoring symptoms, volume status, kidney function, and electrolytes helps clinicians balance benefit and adverse effects.

Rhythm changes the circulation

Atrial fibrillation is an abnormal rhythm in which atrial electrical activity is disorganized. It can impair coordinated filling and produce an irregular ventricular rhythm. Blood can pool in the atria, increasing the risk of clot formation and stroke. NHLBI's overview explains these consequences.

Management considers symptoms, heart rate, rhythm, underlying conditions, and stroke prevention. Anticoagulation decisions require assessment of thromboembolic risk, bleeding risk, contraindications, and preferences. A medicine that slows the pulse does not automatically provide the protection of an anticoagulant. NHLBI's treatment page separates those purposes.

In a fictional patient with palpitations and breathlessness, identifying atrial fibrillation answers the rhythm question. It leaves further questions: what triggered it, how is the circulation tolerating it, is heart failure present, and what long-term protection is appropriate? These remaining questions shape treatment and follow-up.

An arterial pressure is recorded as two numbers. The systolic value is the pressure during ventricular contraction; the diastolic value is the pressure between contractions. Both matter. The familiar reading 120/80 is a pressure measurement in millimeters of mercury, not a ratio to calculate.

Pressure changes with activity, sleep, distress, pain, and treatment. The clinical task is to establish the person's usual pattern and its consequences. A hurried measurement with an unsuitable cuff can send that assessment in the wrong direction before anyone has discussed a diagnosis.

First obtain a usable measurement

The American Heart Association recommends an automatic upper-arm monitor with a suitable cuff. Its home measurement instructions call for at least five minutes of quiet rest, a bare supported arm at heart level, and no smoking, caffeine, or exercise during the preceding thirty minutes. Two readings, a minute apart, provide a more useful record than one. The timing and frequency of longer-term monitoring should be agreed with the clinician.

A seated person should have the back supported, feet on the floor, and legs uncrossed. Talking during the measurement changes the conditions. They help make measurements comparable. The AHA's explanation of the measurement rules describes why they matter.

In a fictional consultation, an initial reading is high after a difficult walk from the car park. A repeat after appropriate rest is lower. Neither number should be erased. The record should distinguish the circumstances. Repeated standardized readings, and sometimes home or ambulatory monitoring, help establish whether the elevation persists.

Why two guidelines can name the same number differently

The 2025 US AHA/ACC guideline calls a systolic pressure of 130–139 or a diastolic pressure of 80–89 stage 1 hypertension. Stage 2 begins at a systolic pressure of at least 140 or a diastolic pressure of at least 90.

The 2024 European Society of Cardiology guideline retains at least 140 systolic or 90 diastolic as the office threshold for hypertension, while introducing an elevated-pressure category below that.

These are different classification systems. A reading of 134/84 does not change physically when it crosses the Atlantic. The labels organize risk assessment and care. A treatment decision also considers repeat measurements, cardiovascular risk, other illness, treatment tolerance, and the applicable guideline. Classification alone does not specify a prescription.

Pressure over years; circulation over minutes

Persistently high blood pressure increases the risk of cardiovascular and kidney disease. Much of its significance concerns damage accumulated over time, even when the person feels well. Follow-up therefore needs a workable long-term plan, including access to medicines where indicated and reliable measurement.

Acute circulatory illness raises another question: are tissues receiving adequate blood flow? A pressure value helps answer it, but symptoms, alertness, skin perfusion, urine output, pulse, and the surrounding illness also matter. Severe new chest pain, breathlessness, collapse, or neurological symptoms require emergency assessment. Repeating home measurements must not delay that response.

Read the record as a sequence

A useful home record includes the date, time, readings, and relevant circumstances. It can reveal disagreement between clinic and everyday measurements. White-coat hypertension describes elevated clinic readings without corresponding elevation outside; masked hypertension describes the reverse. The AHA and American Medical Association's monitoring statement treats home measurement as part of the clinical assessment.

In a case discussion, first establish whether the measurements were sound and what pattern they show. Then consider the appropriate action for that person.

The kidneys continually filter blood, recover substances the body needs, and excrete others. Their work includes controlling water and electrolyte balance, helping regulate blood pressure, supporting red-cell production, and participating in bone and mineral metabolism. A filtration result reports one part of this work.

Each kidney contains many nephrons. Blood is filtered at the glomerulus; the tubule then changes the composition of that filtrate through reabsorption and secretion. Final urine is therefore the result of filtration followed by extensive adjustment. NIDDK's account of kidney function describes this arrangement.

Creatinine needs a time course

Creatinine is used to estimate glomerular filtration rate, usually abbreviated eGFR. The estimate depends on an equation and on assumptions about the relationship between creatinine production and clearance. Muscle mass affects creatinine production. During rapidly changing kidney function, the blood concentration may not yet represent a steady state.

An eGFR of 45 is not a statement that the kidneys are “45 percent working.” It is an estimated filtration rate, commonly reported in milliliters per minute per 1.73 square meters of body surface area. NIDDK's adult equation guidance explains the current race-free equations and their limitations.

Compare two fictional records. In one, creatinine has risen sharply since last week during vomiting and poor intake. In the other, similar abnormal values have been present for a year. The first raises an acute problem; the second establishes a longer history. Either person may have additional complications. A single result cannot supply the missing dates.

Chronic disease has a duration requirement

KDIGO's 2024 guideline defines chronic kidney disease through abnormalities of kidney structure or function present for at least three months, with implications for health. Reduced filtration can establish it, but other findings, including persistent albuminuria, can do so even when filtration is relatively preserved.

This is why assessment commonly uses both an eGFR and a urine albumin-to-creatinine ratio. They describe different features: filtration and abnormal albumin passage. Previous results, urine findings, imaging, and the clinical history help establish chronicity and cause. KDIGO's evaluation summary places these pieces together.

Sodium is a concentration

Low blood sodium often reflects too much water relative to sodium, rather than a simple shortage of dietary salt. The total amount of sodium and water in the body may differ substantially among patients with the same measured concentration. Some are volume depleted; others have edema.

That distinction explains why “drink more” and “eat salt” are poor universal responses to an abnormal sodium result. Clinicians assess the circumstances, medications, glucose, kidney function, and water handling. Severe neurological symptoms can accompany dangerous sodium disturbances. Rapid correction of some chronic disturbances can also injure the brain, so treatment requires an appropriate monitored plan. The Merck professional account of hyponatremia explains the competing problems.

Potassium has electrical consequences

Most potassium is inside cells. The blood measurement samples the much smaller extracellular compartment. Both excessive and insufficient blood potassium can disturb cardiac electrical activity. Kidney function, gastrointestinal losses, medicines, and shifts between cells and blood all help determine the value. Even sample collection can affect a result; repeated fist clenching is one example noted by MedlinePlus.

An unexpected result therefore requires judgment about urgency and reliability. A potentially dangerous value cannot simply be dismissed as a sampling error. Confirmation, an ECG, and treatment decisions depend on the clinical situation.

Follow the connections

Kidney disease may contribute to anemia through reduced erythropoietin production, among other mechanisms. NIDDK's kidney-anemia guide explains why an abnormal blood count can belong in a renal assessment.

Conversely, medicines used for heart disease or diabetes may need review as kidney function changes. The useful question is what this change means for the whole treatment plan: fluid balance, drug handling, electrolytes, symptoms, and follow-up. Previous values and the clinical course help determine the next assessment.

Blood pH describes hydrogen-ion activity on a logarithmic scale. Normal arterial pH is usually about 7.35–7.45. Values below that range indicate acidemia; values above it indicate alkalemia. Acidosis and alkalosis name processes that push pH in those directions. More than one process can be present at once.

The lungs regulate carbon dioxide through ventilation. The kidneys help regulate bicarbonate and acid excretion. The relationship between carbon dioxide and bicarbonate explains the first division in acid–base reasoning: a respiratory process primarily changes carbon dioxide; a metabolic process primarily changes bicarbonate or the balance of nonvolatile acids. The Merck overview sets out this framework.

Start with the direction

Consider this fictional arterial blood gas and chemistry sample:

The pH indicates acidemia. Bicarbonate is low, which fits a metabolic acidosis. Carbon dioxide is also low. Reducing carbon dioxide tends to raise pH, so it is acting in the compensating direction rather than explaining the acidemia.

That sequence matters. Seeing a low carbon dioxide and immediately naming the whole problem “respiratory alkalosis” would miss the dominant disturbance. First examine pH, then ask which change explains its direction, then assess the other change.

Check whether compensation is plausible

For a metabolic acidosis, a commonly used estimate of expected respiratory compensation is Winter's formula:

Expected arterial CO2 pressure = 1.5 × bicarbonate + 8, ± 2 mmHg

With bicarbonate of 12, the estimate is 26, with an expected range of approximately 24–28 mmHg. The measured value, 26, fits that estimate. The formula is presented in the professional review of metabolic acidosis.

A measured carbon dioxide outside the expected range raises the possibility of an additional respiratory disorder. Compensation does not guarantee a normal pH, and a normal pH does not guarantee the absence of disease: opposing primary disorders can partly cancel each other's effect.

These calculations describe a physiological pattern. They do not tell us why it developed or prescribe its correction.

Account for the measured ions

A common anion-gap calculation, omitting potassium, is:

Anion gap = sodium − (chloride + bicarbonate)
          = 140 − (104 + 12)
          = 24 mmol/L

The blood remains electrically balanced. The “gap” represents ions not included in this short calculation, rather than missing electrical charge. A raised value can indicate accumulation of unmeasured anions. Its reference range depends on the laboratory and method, and albumin affects interpretation. MedlinePlus explains the purpose and limits of the test.

In this example the gap is elevated relative to many commonly used ranges. Ketones, lactate, kidney dysfunction, and some toxic exposures are among the possibilities that clinical assessment would investigate. The numbers alone cannot select one.

A second route to low bicarbonate

Not every metabolic acidosis has an increased anion gap. Bicarbonate can be lost through the gastrointestinal tract or through disordered renal handling. Chloride may rise relative to sodium, leaving the calculated gap comparatively unchanged.

Renal tubular acidosis illustrates why filtration and tubular function are different subjects. A kidney can have difficulty with acid handling in ways that are not summarized by an eGFR. NIDDK's explanation describes several forms and their different mechanisms.

Bring the person back into the calculation

Now add a history to the fictional sample: thirst, frequent urination, abdominal discomfort, and rapid breathing in a person with diabetes. Ketoacidosis becomes an urgent possibility. Add a different history—poor circulation during severe infection—and the investigation changes. Add a possible ingestion and it changes again.

The correct interpretation should therefore read something like: “There is acidemia from a metabolic acidosis, with respiratory compensation consistent with the expected range and an increased anion gap. The cause requires urgent clinical assessment.” That statement says what has been demonstrated and leaves the unmeasured cause open.

Keep the calculation beside the history and examination. In a case discussion, explain which additional observation would help distinguish the remaining causes and how urgently it is needed.

Insulin is made by pancreatic beta cells and helps regulate the use and storage of nutrients. Diabetes develops through more than one mechanism. In type 1 diabetes, immune destruction of beta cells usually produces severe insulin deficiency. Type 2 diabetes involves insulin resistance together with an inadequate insulin-secretory response. Age or body size alone cannot reliably determine the type.

NIDDK's type 1 overview describes the autoimmune process. Its research account of diabetes and metabolism discusses the interacting defects involved in type 2 disease. Other forms occur, including diabetes associated with pregnancy, pancreatic disease, medicines, and particular genetic changes.

Diagnose with the correct test and setting

For nonpregnant adults, the American Diabetes Association's 2026 diagnostic standard includes these criteria:

Without unequivocal hyperglycemia, confirmation requires two abnormal results, from the same or different accepted tests, obtained together or at different times as appropriate. Pregnancy has separate diagnostic considerations. A home meter reading is not interchangeable with every laboratory criterion in this table.

These tests establish abnormal glycemia. They do not, by themselves, explain its mechanism or settle the diabetes type. The NIDDK diagnostic guide also distinguishes the tests and the circumstances in which they are used.

A1c measures a history, imperfectly

A1c reflects glucose attached to hemoglobin and is used to estimate average glycemia over roughly three months. It smooths out information: very different daily patterns can produce a similar average. A person with frequent highs and lows may therefore need a different assessment from someone whose glucose remains near that average.

Red-cell lifespan and some hemoglobin variants can alter interpretation. Recent blood loss, transfusion, and other conditions may make the result misleading in a particular person. NIDDK's A1c explanation discusses these limitations. A disagreement between A1c and other glucose measurements deserves investigation rather than automatic preference for one familiar number.

Two acute hyperglycemic crises

Diabetic ketoacidosis involves ketone accumulation and metabolic acidosis. The hyperglycemic hyperosmolar state involves marked hyperglycemia, hyperosmolality, and dehydration. They can overlap. Ketoacidosis can occur without extremely high glucose, including in association with SGLT2-inhibitor medicines.

Blood potassium may initially be normal or high despite depletion of the body's potassium stores. Treatment changes its distribution, which is one reason these illnesses require monitored care. The 2024 international consensus report addresses their diagnosis and management. This chapter does not provide a home insulin or fluid-replacement protocol.

Vomiting, abdominal pain, deep or difficult breathing, severe dehydration, or altered consciousness in a person who may have a hyperglycemic crisis warrants urgent assessment. A glucose value that seems less dramatic than expected cannot safely exclude ketoacidosis.

Treatment can also lower glucose too far

Insulin and some glucose-lowering medicines can cause hypoglycemia. Symptoms may include shakiness, sweating, hunger, confusion, or visual and speech changes. Severe episodes can cause seizures or unconsciousness. People at risk need an individualized recognition and treatment plan, with appropriate emergency supplies and instruction for those around them. NIDDK's hypoglycemia guide explains this planning.

Someone who is unconscious, having a seizure, or unable to swallow safely needs emergency help; do not give food or drink by mouth. A prescribed glucagon rescue product should be used according to the person's emergency plan and its instructions.

Read the follow-up visit broadly

In a fictional review, the A1c has improved, but the person reports overnight sweating, missed meals, and difficulty paying for medication. Calling the visit a success from the A1c alone would discard information that changes the plan.

The clinician needs the actual treatment schedule, glucose pattern, kidney function, symptoms, and practical obstacles. A plan that depends on meals the person cannot reliably obtain is incomplete. The biological mechanism and the daily routine have to be considered together.

Endocrine tests often come in pairs because one hormone regulates the production of another. The measured concentrations make more sense when read as a feedback relationship than as unrelated entries on a laboratory form.

The pituitary releases thyroid-stimulating hormone, or TSH, which stimulates the thyroid. Thyroid hormones then feed back on the regulating system. In primary hypothyroidism, the thyroid fails to provide enough hormone and TSH commonly rises. A low free thyroxine concentration with a high TSH fits that pattern.

Ask where the failure is

If the pituitary or hypothalamic system is failing, a low free thyroxine concentration may be accompanied by a TSH that is low or inappropriately within the laboratory range. A “normal” TSH can therefore be inappropriate for the thyroid hormone concentration beside it. NIDDK's thyroid-testing guide explains the distinction between primary thyroid disease and central causes.

In a fictional exercise, two patients have low free thyroxine. One has a clearly elevated TSH; the other has a TSH within the reference interval and a history suggesting pituitary disease. The same thyroid hormone result leads to different further questions because the feedback response differs.

A useful written interpretation describes both results and their relationship. “TSH normal” is a transcription. “TSH is not appropriately elevated despite low free thyroxine” is an interpretation that still requires clinical confirmation.

Symptoms overlap with other illnesses

Hypothyroidism can produce fatigue, cold intolerance, constipation, and other symptoms, but these complaints are not specific. Their presence prompts assessment; it does not establish a thyroid diagnosis. NIDDK's hypothyroidism overview describes causes, testing, and treatment.

The timing also matters. Long-standing tiredness, a recent medication change, an acute infection, and a newly abnormal laboratory result may not all have one cause. Severe nonthyroidal illness can complicate thyroid-test interpretation. The American Thyroid Association's guide discusses why the clinical circumstances and the combination of tests matter.

A supplement can change the assay

Biotin, sold in some supplements, can interfere with certain laboratory assays. Depending on the method, the result may suggest a hormone abnormality that is not present in the person. This is an analytical effect, not proof that the supplement changed thyroid physiology.

The NIH Office of Dietary Supplements describes interference with thyroid and other tests. The laboratory and clinician need to know what supplements were taken and when. There is no single stopping interval that can be applied to every product, dose, assay, and patient from a paragraph in this book.

The general habit is useful beyond thyroid disease: when results conflict with the clinical picture, consider the sample, assay, medicines, and supplements as well as the diagnosis.

Cortisol and illness

The adrenal glands produce cortisol, which contributes to the body's response to stress and helps maintain several essential functions. Adrenal insufficiency can arise from disease of the adrenal glands or from impaired stimulation by the central hormonal system. Long-term glucocorticoid treatment can suppress that system.

Symptoms may develop gradually, with fatigue, reduced appetite, weight loss, or abdominal symptoms. During an adrenal crisis, circulatory failure and other severe disturbances can develop. NIDDK's account of adrenal insufficiency describes these causes and presentations.

The prescription has a time course too

A person taking long-term steroid treatment should not abruptly stop it without an appropriate medical plan. People with established adrenal insufficiency need individualized instructions for illness, procedures, and emergencies. NIDDK's treatment guidance explains replacement treatment and the need for additional planning during physiological stress.

In a fictional medication history, “steroids” is an incomplete entry. The preparation, dose, duration, recent changes, and reason for treatment all affect interpretation. Someone who recently stopped a prolonged course may present a different problem from someone who used a brief topical preparation.

An endocrine history therefore includes both the body's hormone production and hormones supplied as medicines. The feedback system responds to both. A laboratory value without that history can be correctly measured and still be poorly understood.

Anemia reduces the blood's oxygen-carrying capacity. Hemoglobin concentration is central to its assessment, but the cause is not encoded in the word “anemia.” Red cells may be produced inadequately, lost through bleeding, or destroyed too quickly. More than one process may operate at once.

The NHLBI account of anemia causes includes nutritional deficiencies, blood loss, inherited disorders, inflammation, kidney disease, and treatments that affect production. The history and blood-count pattern help distinguish them.

Read more than the hemoglobin

The complete blood count supplies red-cell measurements alongside white-cell and platelet information. Mean corpuscular volume, or MCV, describes average red-cell size. A low MCV narrows the pattern toward microcytic causes; a high one toward macrocytic causes. Neither is a diagnosis.

Reticulocytes are young red cells. Their number, interpreted in relation to the degree of anemia and the clinical timing, helps assess the marrow's response. A blood film can add information about cell shape and other abnormalities. NHLBI's anemia booklet introduces these investigations; MedlinePlus explains the reticulocyte count in more detail.

A learner can organize the first pass into three questions: how low is the hemoglobin and how is the person tolerating it, what production or loss pattern is suggested, and what cause fits the history? Urgent symptoms or active bleeding can make stabilization the immediate priority before the full explanation is available.

Iron deficiency requires a cause

Consider a fictional adult with hemoglobin of 9 g/dL, equivalent to 90 g/L, small red cells, and a clearly low ferritin. Iron deficiency is strongly supported. The next question is why the stores are low.

Possible explanations include blood loss, inadequate intake, increased requirements, or impaired absorption. Menstrual and gastrointestinal histories may be relevant, depending on the person. Treating the deficiency and investigating its cause are related tasks. NHLBI's iron-deficiency guide discusses both.

Ferritin is useful because it relates to iron stores, but inflammation and some other illnesses can increase it. A result within or above the usual range does not always settle the iron question in an inflamed patient. MedlinePlus's ferritin guide explains these influences. Other iron studies and the clinical setting may be needed.

In the fictional case, recommending an iron-rich meal would not constitute an adequate assessment. The measured anemia, symptoms, and possibility of ongoing loss require a clinical plan.

Oxygen saturation can still look normal

Return to the pulse oximeter in the breathing chapter. It estimates the proportion of available hemoglobin binding sites occupied by oxygen. Anemia reduces the amount of hemoglobin available. A high saturation can coexist with reduced total oxygen carriage.

This explains why breathlessness and fatigue cannot be dismissed solely because a finger device displays a reassuring percentage. The device and the blood count answer different questions. The assessment needs both the symptom history and the relevant physiological measurements.

Platelets and coagulation proteins

Stopping bleeding involves platelets, clotting proteins, blood vessels, and regulation that limits the clot to where it is needed. A platelet count measures quantity; it does not fully measure platelet function. Coagulation tests sample parts of clot formation under laboratory conditions. Results need the bleeding history, medicines, and suspected disorder beside them.

NHLBI's guide to bleeding-disorder diagnosis describes the combination of history, examination, and laboratory testing. A normal result on one routine test cannot exclude every bleeding disorder.

Clotting and bleeding can occur together

Disseminated intravascular coagulation illustrates the interaction. Widespread activation of clotting can consume platelets and clotting factors, while small-vessel clots impair circulation. Bleeding and thrombosis can therefore coexist. It is a serious complication of underlying illness, rather than a simple state of “blood too thin.” NHLBI's DIC overview explains the process.

When reviewing a blood result, keep the components distinct. Hemoglobin concerns oxygen carriage; platelets and coagulation studies concern overlapping aspects of hemostasis; white cells contribute another set of questions. Their combination may reveal a broader illness, with each result interpreted in its own physiological context.

Abdominal symptoms require location, timing, and associated features. Pain, diarrhea, bleeding, difficulty swallowing, and weight loss lead toward different investigations. Their overlap can be informative, but a short symptom label cannot substitute for a history.

In a fictional history, “stomach trouble” initially appears to mean intermittent discomfort. Further questions reveal black stools and lightheadedness. The clinical priority changes. The person's original wording was accurate as a description of distress; the task was to find the details it contained.

Bleeding is a finding to act on

Gastrointestinal bleeding may present as vomiting blood, black tarry stools, or visible blood from the rectum. Smaller chronic losses may first appear through anemia. The location and appearance provide clues, but assessment must also establish the person's stability.

NIDDK's guide to gastrointestinal bleeding advises prompt medical help for its symptoms. Collapse, confusion, severe weakness, or signs of shock make the situation an emergency. The learner's first question is how urgently the person needs care; identifying the precise lesion follows within that care.

Inflammation and absorption

Crohn's disease and ulcerative colitis are forms of inflammatory bowel disease. Crohn's can involve different parts of the digestive tract; ulcerative colitis affects the inner lining of the colon and rectum. Their course and complications require specialist assessment. NIDDK describes Crohn's presentations and ulcerative colitis separately.

Celiac disease raises a different testing problem. Blood testing commonly includes IgA antibodies against tissue transglutaminase, with attention to total IgA because IgA deficiency changes interpretation. Testing should be planned while the person is consuming gluten; removing it first can make the diagnostic evidence harder to obtain. NIDDK's professional testing guide explains the sequence. Further assessment may include intestinal biopsy.

The fictional patient with iron deficiency in the preceding chapter might therefore need questions about digestion and absorption as well as bleeding. An absorption problem can first come to attention through a blood count.

The pancreas has two major jobs

The pancreas supplies digestive enzymes and produces hormones involved in glucose regulation. Pancreatitis concerns inflammation of the organ. Diagnosis brings together the pain history, examination, blood tests such as lipase, and imaging when appropriate. An isolated enzyme result is not a complete diagnosis. NIDDK's pancreatitis diagnostic guide describes these components.

When reviewing an abdominal case, ask what the test was intended to establish. Imaging may investigate anatomy or complications; an enzyme measurement supplies biochemical evidence. One cannot automatically replace the other.

“Liver tests” measure different things

ALT and AST are commonly used markers of cell injury. They do not directly measure how much useful liver function remains. Albumin and clotting measurements can provide information about protein synthesis, while bilirubin relates to production, processing, and excretion. These results also have influences outside the liver.

A low albumin can occur in other illnesses, and an abnormal INR may reflect anticoagulant treatment, vitamin K deficiency, or other causes. The Merck professional guide to liver laboratory tests distinguishes injury, excretion, and synthesis. The practical exercise is to name what each measurement actually samples before combining them into a pattern.

Cirrhosis changes architecture and blood flow

Cirrhosis involves scarring and altered liver structure. Assessment may use the history, blood tests, imaging, and methods that estimate liver stiffness. Complications can include fluid accumulation, portal hypertension, and altered mental function. NIDDK's diagnostic account describes how the evidence is assembled.

For a learner, a useful case summary might separate three questions: what caused the liver disease, what evidence shows its current severity, and which complication is producing today's symptoms? “Cirrhosis” supplies a major diagnosis while leaving all three questions worth asking.

An infection involves a microorganism causing disease. Finding a microorganism in a sample requires a further interpretation: is it causing this illness, colonizing a surface, or present through contamination? The answer depends on the organism, sample site, collection method, symptoms, and patient.

This is why a laboratory report cannot always be read as a direct instruction to prescribe an antibiotic. Record the collection site and method, and the question that prompted the sample.

Locate the suspected illness

A fictional patient arrives with fever and breathlessness. Another has fever and pain passing urine. A third has fever after a procedure. All three need assessment, but their histories suggest different sources and investigations.

The clinical description should retain those differences. “Possible infection” is often an honest early assessment. Adding the suspected site, severity, and relevant exposures makes it useful to the next person caring for the patient. The record should also say what remains uncertain and what would prompt a change in the working diagnosis.

Recognize organ dysfunction

Sepsis is a life-threatening response to infection that can damage organs. It is not synonymous with fever, nor does it require every familiar sign to be present. New confusion, difficulty breathing, abnormal circulation, and deterioration during an infection are concerning features. CDC's sepsis overview emphasizes urgent recognition and care.

Assessment and treatment proceed together when sepsis is suspected. Clinicians look for infection and organ dysfunction, support the circulation and breathing as needed, and give appropriate antimicrobial treatment. They repeatedly review the response and refine therapy as more information becomes available. CDC's clinical guidance stresses immediate evaluation and frequent reassessment.

No single laboratory switch

The 2026 Surviving Sepsis Campaign adult guideline states that sepsis cannot be ruled in or out by a single biomarker or diagnostic test. It recommends early blood cultures, ideally before antimicrobial treatment, and includes lactate measurement in assessment. Testing must fit an urgent care process rather than obstruct it.

The guideline also distinguishes patients with shock or probable sepsis from patients whose likelihood of infection remains uncertain. It addresses early source control when an anatomical source requires it, such as a focus needing a procedure. Clinical reassessment remains necessary after the first treatment has been given.

For the learner, this means that a falling laboratory value can be encouraging without proving the whole illness has resolved. Breathing, circulation, mental state, urine output, the infection source, and treatment effects still require review.

A urine result can distract from the cause

Asymptomatic bacteriuria means bacteria are present in urine without symptoms attributable to urinary infection. It is common in some populations. The Infectious Diseases Society of America's guideline specifically addresses older adults with bacteriuria and delirium but no local urinary symptoms or systemic signs such as fever or hemodynamic instability. It recommends looking for other causes and careful observation rather than automatically giving antibiotics for the urine result.

That recommendation does not tell clinicians to withhold urgent treatment from someone with possible sepsis. It prevents an unrelated positive result from prematurely ending the investigation of a stable person's confusion.

Use the antibiotic for the infection it can treat

Antibiotics do not treat viral infections such as influenza itself. Unnecessary treatment can cause adverse effects and contribute to antimicrobial resistance. CDC's antibiotic-use guide explains why the choice must follow the clinical indication.

A fictional review note might read: “The initial diagnosis was provisional. Culture results and the subsequent examination support a different source, so the treatment plan has been revised.” The note preserves why the initial choice was made and why it changed.

Reassess the suspected cause alongside the response to treatment. Improvement, deterioration, new microbiology, and unexpected side effects each offer reasons to reassess. A treatment decision is part of an ongoing clinical process, with someone responsible for checking what happens next.

The nervous system can produce a focused deficit, such as loss of movement on one side, or a more widespread change in attention and awareness. Timing is often the first clue. A sudden change demands a different response from a slowly progressive difficulty present for years.

The history should establish what the person was like before the change, when they were last known to be at that baseline, and how the symptoms developed. Someone who knows the patient well may provide information the examination cannot recover.

Stroke is time-sensitive

Sudden weakness or numbness, especially on one side, difficulty speaking or understanding speech, visual change, severe imbalance, or a sudden severe headache can signal stroke. Emergency services should be contacted promptly. Symptoms that resolve can still represent an urgent cerebrovascular event. NINDS's symptom guide explains the warning signs.

A person who wakes with symptoms also needs emergency assessment. Record when they were last known well rather than inventing an onset time from when the problem was discovered. Treatment eligibility depends on clinical assessment and imaging; readers should not use a remembered time window to decide that seeking help is pointless.

Imaging changes the treatment question

Stroke can result from blocked blood flow or bleeding. The distinction matters because treatments appropriate for one mechanism may be harmful in the other. Brain and vascular imaging help establish the type and guide treatment. NINDS's assessment and treatment account describes this process.

The learner's task at first recognition is rapid escalation and a clear history. It is not to give an improvised treatment based on the assumption that every stroke is a clot. A concise report of the observed change and last-known-well time is more useful than a confident guess at the vessel involved.

Delirium may be quiet

Delirium involves an acute, often fluctuating disturbance in attention and awareness, with other cognitive changes. Some people are agitated. Others become unusually quiet, sleepy, slow to respond, or less engaged. This hypoactive presentation is easily overlooked.

NICE's delirium guideline calls attention to those quieter changes and to assessment of recent fluctuations. New confusion requires prompt clinical evaluation for an underlying cause. Infection, medicines, metabolic disturbances, pain, and other acute problems may contribute.

An observation such as “pleasantly confused” conceals too much. Better documentation describes the change: unable to sustain attention today, repeatedly losing track of the question, markedly sleepier than yesterday, or fluctuating between clear conversation and disorientation. These descriptions can be compared over time.

Dementia and delirium can coexist

Dementia describes a longer-term decline in cognitive abilities that interferes with everyday function. It does not explain every new change in a person who already has the diagnosis. The National Institute on Aging distinguishes dementia from several other causes of cognitive symptoms.

In a fictional case, a person with established dementia usually recognizes their home and manages a familiar conversation. Over one day they become markedly drowsy and inattentive. The useful question is what changed today. Their baseline disease increases the need for a careful account from someone who knows them; it does not remove the need to investigate an acute illness.

Build a baseline that can be used

For a learning exercise, write a short baseline description before adding a diagnosis: usual alertness, ability to communicate, mobility, and daily assistance. Then write the current findings in the same order. The differences become visible.

This method also avoids equating dependence with confusion. Someone may need help walking while thinking clearly. Someone who speaks fluently may still have difficulty sustaining attention. Description should precede assumptions about capacity or cognition.

Reassess after the immediate event

The care record should preserve which findings were observed, which were reported, and when. “Family reports normal conversation at breakfast” conveys a different kind of evidence from an examination performed that afternoon. Both may be essential.

When several teams become involved, these dates and observations can be lost beneath repeated diagnostic labels. A short chronological account keeps the original question available: what function changed, over what interval, and what evidence now explains it? That account remains useful even as the diagnosis becomes more specific.

A painful joint is a starting description. The examination asks whether the problem is within the joint, in a tendon or nearby structure, or referred from elsewhere. The history asks how quickly it began, whether there is swelling, which joints are involved, and how symptoms affect movement and daily tasks.

For a fictional comparison, one person has gradually increasing discomfort during a particular activity. Another develops a hot, swollen knee over a day. A third has persistent swelling in several small hand joints. The word “arthritis” would obscure useful differences if it were applied before describing them.

A hot joint needs prompt assessment

An acutely swollen, painful joint can have several causes, including crystal disease and infection. Septic arthritis can damage the joint and cause serious illness, so urgent clinical assessment is needed when it is suspected. Examination and joint-fluid analysis may be central to that assessment.

The 2023 guideline for septic arthritis in native joints emphasizes bacterial identification from synovial fluid and cautions that finding crystals does not exclude infection. Two processes can coexist. A familiar history of gout should not automatically explain every new hot joint.

Crystals are evidence of a particular process

Gout results from deposition of monosodium urate crystals. The blood urate concentration contributes information, but the diagnosis is assembled from the presentation and, where appropriate, examination of joint fluid or imaging. NIAMS's gout guide describes the assessment.

Treating an acute attack and managing the longer-term tendency to crystal deposition are different goals. The selection and timing of medicines depend on the person, including kidney function and other treatment. A short introductory chapter cannot assign that regimen from one urate result.

Persistent inflammatory arthritis

Rheumatoid arthritis is assessed through the pattern of joint symptoms and swelling, examination, blood tests, and sometimes imaging. Rheumatoid factor and anti-CCP antibodies can support the diagnosis, but rheumatoid factor is not specific to rheumatoid arthritis, and some affected people have negative tests.

Early recognition matters because treatment aims to control inflammation and reduce damage as well as relieve pain. Disease-modifying medicines have a different purpose from a painkiller used for temporary symptom relief. NIAMS's rheumatoid arthritis guide explains the diagnostic and treatment approach.

In a fictional follow-up, less pain is welcome, but persistent swelling and loss of hand function still require attention. The assessment should record the outcome being measured. Pain, inflammation, structural damage, and ability to work are related, yet they can change at different rates.

An antibody does not name the whole illness

Antinuclear antibodies, usually abbreviated ANA, occur in many people with lupus but can also occur in healthy people. A positive result alone does not diagnose lupus. The history, examination, blood counts, kidney findings, urine tests, and more specific investigations help establish whether a systemic autoimmune disease is present.

NIAMS's lupus diagnostic guide describes that broader assessment. It also explains why treatment follows the manifestations and organ involvement rather than the antibody result in isolation.

This returns us to the probability chapter. Testing people with very different prior likelihoods changes the meaning of a positive result. The interpretation should answer the question that justified the test, rather than converting the laboratory's positive label directly into a disease label.

Record function in ordinary language

A functional history becomes clearer when it names a task: opening a jar, fastening clothes, walking to the bus stop, or completing a work shift. “Doing worse” can mean pain, weakness, stiffness, breathlessness, or difficulty arranging help. Specific descriptions make change easier to judge at the next visit.

For an exercise, write two lines for the same fictional patient. The first might say “hand symptoms improved.” The second might say “can fasten buttons again, but still cannot grip the kettle safely.” The second gives the clinical team a more useful account of improvement and remaining difficulty.

Systemic illness can involve joints, skin, kidneys, blood, and other organs. That breadth makes careful descriptions especially valuable. A series of concrete observations lets a clinician examine whether the findings belong together, while retaining the possibility that more than one condition is present.

Heart failure, diabetes, chronic kidney disease, and arthritis can create overlapping problems for the same person. Medicines interact, appointments compete for time, and one illness can change the treatment options for another. Internal medicine includes the work of making those plans fit together.

NICE's multimorbidity guideline addresses the burden of multiple long-term conditions and their treatment. It emphasizes an individualized approach to care and decisions based on what matters to the person. A technically reasonable recommendation for one disease still needs to be assessed within the whole situation.

Establish what is actually being taken

Medication reconciliation compares the current regimen with the intended regimen and resolves discrepancies, especially during transitions such as hospital admission or discharge. Missing medicines, duplicate treatment, and unintended changes can arise when records do not agree. AHRQ's PSNet primer describes the process and notes that reconciliation alone does not solve every problem after discharge.

The list should include prescription medicines, nonprescription products, supplements, inhalers, drops, creams, and injections where relevant. It should distinguish what was prescribed from what is actually taken. AHRQ's MATCH toolkit provides a structured approach to medication histories and transitions.

In a fictional example, an electronic record lists one tablet strength, the discharge letter lists another, and the person has both containers at home. Copying either list without resolving the difference leaves the practical problem intact. Someone must establish the intended plan and communicate it clearly.

A new symptom may follow a change

When a symptom begins, compare its date with new medicines, dose changes, supplements, and interruptions in treatment. This does not prove causation. It identifies a relationship worth examining.

A useful history says, “The dizziness began two days after the change, occurs on standing, and has caused one near-fall.” It also records the actual regimen and relevant measurements. “Medication side effect” is a hypothesis until the assessment supports it.

The response may involve adjustment, monitoring, or a different explanation entirely. Some medicines require tapering, and abrupt withdrawal can be harmful. The learner's role is to recognize the need for a review, not to improvise a stopping order from a temporal association.

State the benefit in absolute terms

Consider an invented study example, unrelated to any actual medicine. Over five years, an event occurs in 20 of every 100 people without treatment and 15 of every 100 with treatment. The absolute reduction is five percentage points. The relative reduction is 25 percent, because five is one quarter of twenty.

Now imagine the same relative reduction in a group with a baseline risk of four in a hundred. The treated risk would be three in a hundred, giving an absolute reduction of one percentage point. These are arithmetic illustrations, not predictions for a real patient.

The time period belongs in the sentence. A five-year risk cannot be compared casually with a one-year risk. Nor does a benefit estimate describe the adverse effects, treatment burden, or uncertainty around the study result. Those require their own account.

Ask which outcomes matter

The NICE recommendations call for discussion of disease burden, treatment burden, goals, and priorities before an individualized plan is agreed. Independence, symptoms, adverse effects, prevention of future events, and the number of appointments can all matter.

In a fictional review, the person's most pressing concern is that morning treatment and travel to separate appointments leave little time for paid work. That does not automatically settle the medical decisions. It supplies a constraint the plan must address if it is to be usable.

End with named responsibilities

A plan should identify the intended changes, who will check the response, which results remain pending, and how the person can obtain help if the situation changes. “Follow up” is less useful than a clear arrangement understood by the people responsible.

For a learning exercise, inspect a fictional discharge summary and circle every future action. If there is no person or service responsible for an action, the summary has left a gap. If a medicine changed without an explanation, the next clinician may have difficulty determining whether the change was deliberate.

Before the person leaves, check that the instructions and arrangements agree. Give the patient the agreed plan and make it available to the next clinical team.

These cases are invented teaching exercises. They contain selected details to practice interpretation and are not complete clinical assessments. In real care, urgent treatment and further investigation depend on findings that a short written case may omit.

For each case, first write what the information establishes. Then write what remains uncertain. This separates a defensible interpretation from a diagnosis supplied by habit.

Case 1: a positive screening result

An imaginary test is 90 percent sensitive and 95 percent specific. In a group of a thousand people, twenty have the condition. Eighteen of those twenty test positive. Among the 980 without the condition, forty-nine test positive.

How many positive results are true positives?

There are sixty-seven positives in total: eighteen true and forty-nine false. The proportion that is true is 18 divided by 67, or approximately 27 percent. Sensitivity did not change. The low prevalence made false positives numerous relative to true positives.

The next step for a real positive result would depend on the actual condition, test, consequences, and confirmation pathway. The arithmetic demonstrates why the statement “the test is 90 percent sensitive, so a positive result means a 90 percent chance of disease” is incorrect.

Case 2: breathlessness with a normal saturation

A fictional adult has progressive exertional breathlessness and fatigue. A finger pulse oximeter displays 98 percent. The blood count shows hemoglobin of 9 g/dL, and further assessment supports iron deficiency.

Does the saturation exclude a problem with oxygen carriage?

No. Saturation estimates how much of the available hemoglobin is occupied by oxygen. Hemoglobin concentration describes how much carrier is present. The two results can coexist. The anemia needs evaluation of severity, tolerance, and cause. Other causes of breathlessness may also coexist and should not be excluded merely because anemia has been found.

The careful answer links the measurements to physiology while leaving the rest of the assessment open. It does not use one abnormal result to explain every symptom automatically.

Case 3: a changed creatinine

A patient has a reduced eGFR on today's report. There are no previous results in the referral letter. The patient reports several days of vomiting.

Can the report establish chronic kidney disease?

It cannot establish chronicity by itself. Prior measurements, the course of illness, urine findings, imaging where indicated, and subsequent assessment may help distinguish acute and chronic processes. Both can occur together. The vomiting adds a possible acute contributor, but it does not prove that volume depletion is the only cause.

The immediate assessment also asks about urine output, circulation, electrolytes, medicines, and other complications. Looking for older records should not delay attention to an acute problem.

Case 4: the acid–base sample

The arterial pH is 7.29, bicarbonate 12 mmol/L, carbon dioxide pressure 26 mmHg, sodium 140 mmol/L, and chloride 104 mmol/L.

Write the interpretation before naming a disease.

There is acidemia. Low bicarbonate supports a metabolic acidosis. The low carbon dioxide is in the compensating direction. Winter's formula gives an expected carbon dioxide of 26, plus or minus 2 mmHg, so the measured value fits. The anion gap, calculated without potassium, is 24 mmol/L.

This pattern directs further investigation toward causes of increased-gap metabolic acidosis in the clinical setting. It does not distinguish ketoacidosis, lactate accumulation, renal causes, or certain toxic exposures on its own. The cause and the person's stability determine the urgent management plan.

Case 5: quiet confusion and a urine culture

An older adult is newly inattentive and withdrawn. A urine culture contains bacteria. There are no local urinary symptoms, fever, or signs of hemodynamic instability in the information supplied.

Does the culture prove the confusion is a urinary infection?

It does not. Acute confusion needs assessment for its causes. Under the circumstances described, the IDSA guidance discussed in the infection chapter advises investigation of other causes and careful observation rather than automatically treating bacteriuria. A new medicine, metabolic disturbance, pain, or another illness may be relevant.

If systemic illness or possible sepsis emerges, the assessment and treatment priorities change. The original answer depends on the conditions stated in the case; it cannot be extended to a deteriorating patient by removing those conditions.

Case 6: a high TSH and a low free thyroxine

One fictional patient has a high TSH and low free thyroxine. Another has low free thyroxine with a TSH inside the laboratory interval, alongside evidence that raises concern about pituitary disease.

Why is the second TSH not necessarily reassuring?

A feedback signal should be interpreted relative to the hormone it regulates. In the second pattern, the TSH may be inappropriately low for the low thyroid hormone concentration. The clinical history, assay issues, medicines, illness, and further endocrine assessment matter. The reference interval alone cannot judge whether the response is physiologically appropriate.

Case 7: the discharge list

A fictional patient leaves hospital with a changed medicine regimen. The discharge letter lists the new plan, but the old medicines remain in the kitchen and the primary-care record has not been updated.

What is unfinished?

The intended regimen has been written, but the discrepancy has not been resolved across the places where treatment happens. The patient needs clear instructions; the relevant clinicians need the updated list and reasons for changes; follow-up and pending results need responsible people. The solution may require a pharmacist or another member of the care team as well as the prescriber.

The unresolved task is communication and coordination. It should have a named owner and a check that the updated instructions have been understood.

Continue from a question you can state

When returning to a chapter, begin with one question: what does this measurement mean, which mechanism explains this pattern, or what information would change the decision? Use the linked sources to extend that question. Check the source date and whether the guidance applies to the population and setting you are studying.

In a learning note, record the finding, the interpretation it supports, and an observation that could challenge it. As more evidence arrives, revise the interpretation. The original finding should remain visible, with its date and circumstances, so that the next person can understand how the conclusion was reached.