Lab Interpretation

01 Reference Ranges & Critical Values

Laboratory interpretation is the core cognitive skill that translates a column of numbers into a diagnosis, a treatment, or an urgent phone call. Every lab result must be interpreted in the context of the patient, the pre-test probability, the assay characteristics, and the timing of the measurement. A number outside the reference range is not automatically pathologic; a number inside the reference range is not automatically reassuring.

Reference Range Definitions

A reference range is typically defined as the central 95% of values from a healthy reference population — meaning that by definition, 5% of healthy individuals will fall outside any given reference interval. When multiple independent tests are ordered (e.g., a 14-analyte CMP), the probability that at least one result will fall outside the reference range purely by chance exceeds 50%. This statistical reality is why interpretation must always be anchored to clinical pre-test probability, not to reference range flags alone.

Critical Values Requiring Immediate Action

The Gaussian Trap

Many lab values are not normally distributed — they are skewed (ferritin, triglycerides, liver enzymes, hormones). Reference intervals derived by assuming a Gaussian distribution may misrepresent the true 2.5th and 97.5th percentiles, leading to overcalling "abnormality" on one tail and underestimating it on the other. Log-transformed reference intervals are more accurate for skewed analytes, but not every lab reports them. Clinically, this means that a ferritin of 15 ng/mL may be flagged "normal" while already representing iron depletion, and an ALT of 35 U/L may be flagged "normal" while being inappropriately high for a thin, healthy young woman.

Age, Sex & Population-Specific Ranges

Units: Conventional vs SI

US laboratories typically report in conventional units (mg/dL, g/dL, μg/mL), while most of the world uses SI units (mmol/L, g/L, μmol/L). Common conversions worth memorizing: creatinine mg/dL × 88.4 = μmol/L; glucose mg/dL × 0.0555 = mmol/L; calcium mg/dL × 0.25 = mmol/L; bilirubin mg/dL × 17.1 = μmol/L. Misreading the units (mg/dL vs mg/L) is a classic source of 10-fold dosing errors.

02 Test Characteristics & Bayesian Reasoning

Every diagnostic test has four fundamental characteristics that govern its clinical utility: sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV). Sensitivity and specificity are properties of the test itself; predictive values depend on the prevalence (pre-test probability) of disease in the population being tested.

The Core Definitions

Bayesian Reasoning in Practice

Post-test probability = (pre-test odds × likelihood ratio) converted back to probability. The clinical implication: a highly sensitive and specific test is useless when applied to a population with extremely low pre-test probability, because even a small false-positive rate will produce more false positives than true positives. This is the mathematics behind "don't order a d-dimer on a patient with PERC-negative low-probability PE" and "don't screen 20-year-olds for prostate cancer."

Sources of Diagnostic Error

Common Bayesian errors include base rate neglect (forgetting that a rare disease remains rare even after a positive test), anchoring (sticking with an initial impression despite contrary data), and the representativeness heuristic (matching the patient to a classic vignette while ignoring prevalence). The correct approach is to explicitly estimate pre-test probability before ordering the test, choose the test whose likelihood ratio will move probability across an action threshold, and then update.

Screening vs Diagnostic Testing

A screening test is applied to an asymptomatic population and should prioritize high sensitivity (minimizing false negatives, accepting more false positives which will be adjudicated by confirmatory testing). A diagnostic test is applied to a symptomatic or higher-risk population and should prioritize high specificity and positive likelihood ratio. Mismatching the test to the context (ordering screening tests in high-prevalence populations, or diagnostic tests in asymptomatic people) produces predictable errors. For example, HIV screening uses a highly sensitive ELISA; a positive is confirmed with a specific Western blot or differentiation assay. Troponin in the emergency department is used diagnostically, with serial changes at tight thresholds.

Reflex & Cascade Testing

Many labs run reflex algorithms: a positive HIV screen automatically triggers confirmatory testing; a high TSH reflexes to free T4; a positive hepatitis C antibody reflexes to HCV RNA. Understanding these algorithms prevents unnecessary duplicate orders and ensures appropriate follow-up. Cascade testing (one abnormal result triggering an ever-widening workup) must be resisted unless each step is clinically justified — otherwise a single borderline value snowballs into thousands of dollars of testing.

03 Pre-Analytic Errors & Specimen Integrity

The majority of laboratory errors occur not in the analyzer but in the pre-analytic phase — during specimen collection, handling, labeling, and transport. A hemolyzed sample, a mislabeled tube, or a delayed transport can invalidate a result entirely or produce a spuriously abnormal value that triggers unnecessary intervention.

Tube Color, Additive & Purpose

Hemolysis, Lipemia & Icterus

Hemolysis releases intracellular contents into serum, spuriously elevating potassium, LDH, AST, phosphate, and magnesium, while decreasing sodium by dilution. Potassium in particular can rise 0.5–1.0 mmol/L with visible hemolysis and far higher with severe hemolysis. Never treat "hyperkalemia" on a hemolyzed sample without a repeat draw — the classic error is pushing calcium gluconate and insulin on a patient with a pristine heart and a pristine serum potassium.

Lipemia (turbid post-prandial sample or hypertriglyceridemia) interferes optically, falsely depressing sodium (pseudohyponatremia) on certain analyzers, and distorting many spectrophotometric assays. Icterus (high bilirubin) interferes with creatinine by Jaffe method, lipids, and total protein.

Classic Pre-Analytic Traps

Spurious Results to Recognize Immediately

04 RBC Indices & the Anemia Workup

Anemia is defined by the WHO as hemoglobin <13 g/dL in men or <12 g/dL in women. The foundational move in any anemia workup is to classify by mean corpuscular volume (MCV) into microcytic (<80 fL), normocytic (80–100 fL), or macrocytic (>100 fL), then to integrate reticulocyte response and peripheral smear findings. Getting this right the first time avoids the costly habit of ordering scattershot iron/B12/folate/SPEP panels on every anemic patient.

The RBC Indices

Microcytic Anemia (MCV <80)

Normocytic Anemia (MCV 80–100)

Divide by reticulocyte index. A low retic index (<2%) suggests hypoproliferation: anemia of chronic disease, chronic kidney disease (low erythropoietin), early iron deficiency, aplastic anemia, marrow infiltration, and endocrine disease. A high retic index (>2%) suggests either hemolysis or acute blood loss. Key tests: LDH, haptoglobin, indirect bilirubin, peripheral smear, direct antiglobulin test (Coombs), creatinine, EPO level.

Macrocytic Anemia (MCV >100)

05 Iron Studies, B12 & Folate

Iron studies are the most commonly misinterpreted panel in hematology. The five components are serum iron, total iron-binding capacity (TIBC), transferrin saturation (iron/TIBC × 100), ferritin, and sometimes soluble transferrin receptor. Each has pitfalls.

Interpretation Framework

B12 & Folate

Vitamin B12 (cobalamin) is absorbed in the terminal ileum bound to intrinsic factor secreted by parietal cells. Causes of deficiency include pernicious anemia (autoimmune parietal cell destruction), gastrectomy, terminal ileal disease (Crohn's, resection), bacterial overgrowth, tapeworm (Diphyllobothrium latum), strict vegan diet, and chronic PPI/metformin use. Folate deficiency occurs in malnutrition, alcoholism, pregnancy, hemolysis, methotrexate, trimethoprim, and phenytoin.

06 Reticulocytes & Hemolysis Labs

The reticulocyte count is the single most important test for classifying anemia: it tells you whether the marrow is responding appropriately to the drop in hemoglobin. A raw reticulocyte percentage must be corrected for the degree of anemia, because a 2% retic count in a patient with Hct 15 represents a failed marrow, not a robust response.

Reticulocyte Index Calculation

Absolute reticulocyte count = retic% × RBC count. Corrected retic count = retic% × (patient Hct / 45). Reticulocyte production index (RPI) = corrected retic / maturation factor (1.0 at Hct 45, 1.5 at 35, 2.0 at 25, 2.5 at 15). An RPI >2 suggests appropriate marrow response (hemolysis or acute blood loss); RPI <2 suggests hypoproliferation.

Hemolysis Lab Panel

Intravascular vs Extravascular

Intravascular hemolysis (mechanical fragmentation, complement-mediated lysis, G6PD with oxidant stress) produces hemoglobinemia, hemoglobinuria, severely reduced haptoglobin, and schistocytes on smear. Extravascular hemolysis (splenic destruction of coated or abnormally shaped RBCs) produces splenomegaly, more modest haptoglobin drop, and spherocytes or other abnormal shapes.

07 WBC Differential & Leukocyte Disorders

The total WBC count is less informative than the differential. A normal WBC with a profound left shift, a normal WBC with severe lymphopenia, or an "elevated" WBC that is all mature neutrophils in an exercised patient each tell very different stories.

Normal Differential

Neutrophilia & Left Shift

Neutrophilia (>7.5 ×10⁹/L) reflects infection, inflammation, stress (catecholamines demarginate neutrophils), glucocorticoids, smoking, hematologic malignancy (CML, chronic neutrophilic leukemia), or myeloproliferative disease. A left shift means increased band forms and immature neutrophils (metamyelocytes, myelocytes) in the peripheral blood — a marker of accelerated marrow release typical of severe infection. Toxic granulation, Döhle bodies, and cytoplasmic vacuoles are morphologic features of neutrophils responding to severe bacterial infection.

Neutropenia

Causes include chemotherapy, radiation, drugs (clozapine, methimazole, PTU, sulfa, carbamazepine), viral infection (HIV, EBV, CMV, parvovirus), autoimmune disease, B12/folate deficiency, hypersplenism, congenital syndromes (cyclic neutropenia, Kostmann), and aplastic anemia. Benign ethnic neutropenia is common in patients of African descent and does not confer infection risk.

Lymphocytosis, Monocytosis, Eosinophilia

08 Platelets & Pancytopenia

Normal platelet count is 150–450 ×10⁹/L. Thrombocytopenia is defined as <150, with bleeding risk meaningful <50 and spontaneous bleeding risk substantial <20.

Thrombocytopenia Differential

Thrombocytosis

Platelet counts >450 can be reactive (iron deficiency, infection, inflammation, malignancy, post-splenectomy, post-hemorrhage) or primary (essential thrombocythemia, polycythemia vera, CML, primary myelofibrosis). Reactive thrombocytosis rarely requires intervention. Primary thrombocytosis with counts >1000 carries both bleeding and thrombotic risk and is worked up with JAK2, CALR, and MPL mutations.

Pancytopenia

Simultaneous reduction in all three cell lines points to marrow failure or marrow replacement. Differential: aplastic anemia, MDS, acute leukemia, myelofibrosis, marrow infiltration (metastatic cancer, lymphoma, granulomas), megaloblastic anemia (B12/folate), hypersplenism, PNH, drugs, sepsis, SLE, HIV. Peripheral smear plus bone marrow biopsy are the definitive next steps.

09 Peripheral Blood Smear Findings

The peripheral smear remains one of the highest-yield tests in medicine. A single slide can establish the diagnosis of TTP, sickle cell crisis, malaria, acute leukemia, hereditary spherocytosis, or lead poisoning, each of which might be delayed or missed by reliance on indices alone.

RBC Morphology

WBC & Platelet Morphology

Parasites & Inclusions

Malaria species ID on thin smear is based on infected RBC size, presence of Schuffner's stippling (P. vivax/ovale), banana-shaped gametocytes (P. falciparum), and parasitemia percentage — >5% parasitemia with falciparum is severe malaria and warrants IV artesunate and consideration of exchange transfusion.

10 PT/INR, aPTT, TT & Mixing Studies

The standard coagulation panel measures three pathways: PT (prothrombin time, reported as INR) tests the extrinsic and common pathways (factors VII, X, V, II, fibrinogen); aPTT (activated partial thromboplastin time) tests the intrinsic and common pathways (XII, XI, IX, VIII, X, V, II, fibrinogen); TT (thrombin time) tests the final conversion of fibrinogen to fibrin.

Normal Ranges

The PT/aPTT Matrix

Mixing Studies

When PT or aPTT is prolonged, a mixing study (1:1 mix with normal plasma) distinguishes factor deficiency from inhibitor. If the prolonged time corrects to normal after mixing, the patient has a factor deficiency (the normal plasma supplied the missing factor). If it fails to correct, an inhibitor is present — most commonly a lupus anticoagulant or an acquired factor inhibitor (classically factor VIII inhibitor in acquired hemophilia).

11 Anticoagulant Monitoring & DIC

Warfarin

Warfarin inhibits vitamin K epoxide reductase, depleting functional factors II, VII, IX, X and proteins C and S. Monitor with INR. Standard target 2.0–3.0 for most indications (AFib, VTE, tissue valves); 2.5–3.5 for mechanical mitral valves and some antiphospholipid syndrome. INR >5 without bleeding: hold doses and consider low-dose oral vitamin K. INR with major bleeding: 4-factor PCC (preferred) plus IV vitamin K; FFP is second-line when PCC unavailable.

Heparin

DOACs

Direct oral anticoagulants (apixaban, rivaroxaban, edoxaban, dabigatran) do not require routine monitoring. Apixaban and rivaroxaban are factor Xa inhibitors; dabigatran is a direct thrombin inhibitor. When levels are needed (before surgery, in major bleeding, in extremes of weight or renal function): drug-specific anti-Xa assays for Xa inhibitors; diluted thrombin time or ecarin clotting time for dabigatran. Reversal agents: andexanet alfa for apixaban/rivaroxaban; idarucizumab for dabigatran.

DIC

12 Thrombophilia, vWD & HIT

Hereditary Thrombophilia Workup

Do not order thrombophilia testing during acute thrombosis or while anticoagulated unless you understand the caveats; many results will be falsely abnormal. The clinical impact is also limited — most testing does not change duration of anticoagulation for a first provoked VTE.

Antiphospholipid Syndrome (APS)

Requires clinical criterion (thrombosis or pregnancy morbidity) plus lab criterion: lupus anticoagulant, anti-cardiolipin IgG/IgM, or anti-β2-glycoprotein I, positive on two occasions >12 weeks apart. The lab triad of prolonged aPTT, positive lupus anticoagulant, and positive anti-cardiolipin defines triple positivity and the highest-risk phenotype.

von Willebrand Disease

Heparin-Induced Thrombocytopenia (HIT)

The 4Ts score stratifies pre-test probability: Thrombocytopenia (magnitude), Timing (day 5–10 of heparin, or <1 day with recent exposure), Thrombosis or other sequelae, and oTher causes. Score 0–3 = low (<5% probability); 4–5 = intermediate; 6–8 = high. In intermediate or high pre-test probability, send anti-PF4/heparin antibodies (ELISA) and confirm positives with functional assay (serotonin release assay). Stop all heparin (including flushes) and start a non-heparin anticoagulant (argatroban, bivalirudin, fondaparinux).

Factor Assays & Specific Deficiencies

TEG & ROTEM

Viscoelastic testing (thromboelastography or rotational thromboelastometry) provides a global assessment of clot formation, kinetics, strength, and fibrinolysis in whole blood. Increasingly used in trauma, obstetric hemorrhage, and cardiac surgery to guide goal-directed transfusion: prolonged R time (initiation) suggests need for FFP or factor concentrate; low alpha angle or MA suggests need for fibrinogen or platelets; increased LY30 suggests hyperfibrinolysis warranting tranexamic acid.

13 Sodium, Potassium & Chloride

Sodium & Tonicity

Sodium disorders are fundamentally disorders of water, not of sodium content. Classify hyponatremia by serum osmolality first, then by volume status, then by urine sodium and urine osmolality.

Hyponatremia causes by category: hypovolemic hypotonic — GI losses, diuretics, adrenal insufficiency, cerebral salt wasting. Euvolemic hypotonic — SIADH, hypothyroidism, glucocorticoid deficiency, primary polydipsia, exercise-associated hyponatremia. Hypervolemic hypotonic — heart failure, cirrhosis, nephrotic syndrome, advanced renal failure.

Hypernatremia

Always implies a water deficit (inability to access water or unable to concentrate urine). Causes: diabetes insipidus (central or nephrogenic), osmotic diuresis, insensible losses in an unconscious patient, hypertonic saline or feedings, salt poisoning. Free water deficit formula: TBW × (Na/140 − 1), where TBW = 0.6 × kg in men, 0.5 × kg in women.

Potassium

EKG progression in hyperkalemia: peaked T waves (~5.5–6.5) → PR prolongation, loss of P waves (~6.5–7.5) → wide QRS, sine wave (>8) → VF or asystole. Treat with calcium (membrane stabilization), insulin + glucose (shift), beta-agonists (shift), bicarbonate (shift, if acidotic), and removal (loop diuretics, dialysis, potassium binders).

Chloride & Bicarbonate

Chloride typically tracks with sodium but diverges in acid-base disorders. Low chloride with high bicarbonate suggests metabolic alkalosis (vomiting, diuretics); high chloride with low bicarbonate suggests normal anion gap metabolic acidosis (diarrhea, RTA, saline resuscitation).

14 Calcium, Magnesium & Phosphate

Calcium

Total calcium (normal 8.5–10.5 mg/dL) reflects both albumin-bound and free (ionized) calcium. Ionized calcium (normal 1.1–1.3 mmol/L) is the physiologically active form. Correct total calcium for albumin: corrected Ca = measured Ca + 0.8 × (4 − albumin). In critical illness, order ionized calcium directly.

Hypocalcemia causes: hypoparathyroidism (post-thyroidectomy, autoimmune), vitamin D deficiency, chronic kidney disease, hypomagnesemia (impairs PTH release), acute pancreatitis, sepsis, citrate toxicity after massive transfusion. Symptoms include perioral numbness, Chvostek and Trousseau signs, tetany, QT prolongation, and seizures.

Magnesium

Normal 1.7–2.4 mg/dL. Hypomagnesemia is common in alcoholism, diarrhea, PPIs, loop and thiazide diuretics, aminoglycosides, refeeding syndrome. Clinically, hypomagnesemia causes refractory hypokalemia (renal K wasting) and refractory hypocalcemia (impaired PTH release) — always replace Mg before or alongside K and Ca in ICU patients. Hypermagnesemia occurs almost exclusively in renal failure or iatrogenic (eclampsia treatment); presents with hyporeflexia and respiratory depression at >10 mg/dL.

Phosphate

Normal 2.5–4.5 mg/dL. Hyperphosphatemia: renal failure, tumor lysis, rhabdomyolysis, hypoparathyroidism. Hypophosphatemia: refeeding, DKA recovery, alcoholism, respiratory alkalosis, phosphate binders, severe burns. Severe hypophosphatemia (<1 mg/dL) causes muscle weakness, respiratory failure (diaphragm), rhabdomyolysis, and hemolysis.

15 Renal Function: BUN, Creatinine & eGFR

Creatinine is produced from muscle at a relatively constant rate and cleared by glomerular filtration with a small tubular secretion component. It is the workhorse marker of renal function but is insensitive to early GFR decline and depends on muscle mass, age, sex, and ethnicity.

eGFR Equations

CKD Staging

BUN:Creatinine Ratio

16 AKI vs CKD & Urine Indices

KDIGO AKI Staging

Pre-Renal vs Intrinsic vs Post-Renal

Formula: FENa = (U_Na × P_Cr) / (P_Na × U_Cr) × 100. FEUrea is preferred when diuretics are on board because they artificially raise urine sodium and invalidate FENa.

Intrinsic AKI Urinary Findings

17 Systematic ABG Interpretation

Every ABG should be interpreted with the same five-step algorithm. Skipping steps is the primary reason clinicians miss mixed disorders.

Primary Disorders & Expected Compensation

If the measured PaCO₂ is higher than Winter's predicted value, a concurrent respiratory acidosis is present. If lower, a concurrent respiratory alkalosis is present. This is how you detect mixed disorders hidden in an apparently simple acidosis.

Oxygenation

The A–a gradient quantifies gas exchange. PAO₂ = FiO₂(P_atm − 47) − PaCO₂/0.8. On room air at sea level, PAO₂ ≈ 150 − PaCO₂/0.8. Normal A–a gradient < (age/4) + 4. An elevated A–a gradient indicates V/Q mismatch, shunt, or diffusion impairment; a normal A–a gradient with hypoxemia suggests hypoventilation or low FiO₂.

The P/F Ratio

PaO₂/FiO₂ ratio is used for ARDS classification: mild 200–300, moderate 100–200, severe <100 (Berlin definition, with PEEP ≥5 cm H₂O). A patient with PaO₂ 80 on room air (P/F = 381) is very different from a patient with PaO₂ 80 on FiO₂ 1.0 (P/F = 80) even though the PaO₂ is identical.

18 Anion Gap, Delta-Delta & Mixed Disorders

Anion Gap

Anion gap = Na − (Cl + HCO₃). Normal 8–12 mmol/L. Correct for albumin: add 2.5 per 1 g/dL decrease below 4. A patient with albumin 2 has a "corrected" normal AG of 13.

HAGMA — MUDPILES and GOLD MARK

Non-Gap (Hyperchloremic) Acidosis — HARDUPS

Hyperalimentation (TPN), Acetazolamide, RTA (types 1, 2, 4), Diarrhea, Ureterosigmoidostomy, Pancreatic fistula, Saline (large-volume normal saline resuscitation). Distinguish diarrhea from RTA with urine anion gap: UAG = (U_Na + U_K) − U_Cl. Negative UAG (−20 to −50) suggests appropriate renal ammonium excretion (diarrhea, GI loss). Positive UAG suggests impaired renal acidification (RTA).

Delta-Delta & Delta Gap

Delta ratio = (AG − 12) / (24 − HCO₃). Interpretation: <0.4 pure non-gap acidosis; 0.4–1 mixed gap and non-gap; 1–2 pure gap acidosis; >2 concurrent metabolic alkalosis or pre-existing chronic respiratory acidosis (compensated high HCO₃).

Metabolic Alkalosis: Chloride-Responsive vs Unresponsive

19 Liver Function Tests

"LFTs" are a misnomer — AST, ALT, ALP, and GGT are markers of hepatocyte injury or cholestasis, not function. True liver function is measured by synthetic markers: albumin, PT/INR, bilirubin (with caveats), and ammonia.

Normal Ranges & Cellular Source

Hepatocellular vs Cholestatic Pattern

AST:ALT Ratio

AST:ALT ratio >2 with both <300 U/L is classic for alcoholic hepatitis ("Scotch & toast" — AST for alcohol). AST:ALT >1 in non-alcoholic disease suggests cirrhosis or muscle source. ALT >AST is typical of most other hepatocellular injury (viral, NAFLD early, drug). Transaminases >1000 narrow the differential to ischemic hepatitis, acute viral hepatitis, drug (acetaminophen), autoimmune hepatitis, and Wilson's.

Bilirubin Fractionation

Indirect (unconjugated) hyperbilirubinemia: hemolysis, ineffective erythropoiesis, Gilbert's syndrome, Crigler-Najjar. Direct (conjugated) hyperbilirubinemia: biliary obstruction, hepatocellular disease, Dubin-Johnson, Rotor syndrome, sepsis. A direct fraction >50% of total bilirubin always implies hepatobiliary disease.

Disease-Specific Serology

Child-Pugh & MELD

Child-Pugh score uses bilirubin, albumin, INR, ascites, and encephalopathy to stratify cirrhosis (A/B/C). MELD score uses bilirubin, INR, creatinine, and sodium to prioritize liver transplant allocation: MELD-Na ≥15 is the traditional threshold for listing. Each component has pitfalls — INR differs across labs (ISI variability), creatinine underestimates dysfunction in sarcopenic patients, and bilirubin can be artifactually elevated from hemolysis.

20 Pancreatic & Cardiac Markers

Pancreatic Enzymes

Lipase (normal <160 U/L) is more specific for pancreatitis than amylase and remains elevated longer (7–14 days vs 3–5 days for amylase). Diagnosis of acute pancreatitis requires 2 of 3: characteristic abdominal pain, lipase >3× ULN, and imaging findings. The magnitude of lipase elevation does not correlate with severity. Amylase is less useful: elevated also in parotitis, small bowel obstruction, mesenteric ischemia, ruptured ectopic pregnancy, DKA, and renal failure.

Cardiac Markers

Troponin Elevation — Not Always MI

Troponin is cardiac-specific but not MI-specific. Elevation occurs in myocarditis, pericarditis, Takotsubo, sepsis, PE, heart failure exacerbation, renal failure, strenuous exercise, chemotherapy cardiotoxicity, tachyarrhythmia, and trauma. The Fourth Universal Definition of MI requires rise and/or fall of troponin with at least one value above the 99th percentile, plus ischemic symptoms, new ischemic ECG changes, new wall motion abnormality, or angiographic evidence of thrombus.

BNP & NT-proBNP Interpretation

BNP is lower in obesity (adipose clearance) and higher in women, elderly, AFib, PE, pulmonary HTN, sepsis, and CKD. BNP is cleaved by neprilysin, so patients on sacubitril/valsartan will have artificially elevated BNP; follow NT-proBNP instead.

Other Cardiac & Inflammatory Markers

21 Lipids & Thyroid Studies

Lipid Panel

Friedewald formula: LDL = TC − HDL − TG/5. Invalid when TG >400 or in type III dyslipidemia. For LDL <70, direct LDL is more accurate.

Thyroid Function Tests

Thyroid Antibodies

TPO antibody (anti-thyroid peroxidase) — Hashimoto's thyroiditis (positive in ~95%) and Graves (~75%). TSH receptor antibody (TRAb/TSI) — Graves disease, useful in pregnancy and ambiguous presentations. Thyroglobulin antibody — present in Hashimoto's, can interfere with thyroglobulin tumor marker assays in thyroid cancer follow-up.

22 Diabetes & Glycemic Markers

Diagnostic Criteria (ADA)

HbA1c Pitfalls

HbA1c reflects average glucose over the prior 8–12 weeks but is unreliable in conditions that alter RBC lifespan: hemolytic anemia (falsely low), recent blood transfusion (dilutional), iron deficiency anemia (falsely high), CKD, hemoglobinopathies (HbS, HbC, HbE — depends on assay), and pregnancy. In these situations use fructosamine (2–3 week window) or continuous glucose monitoring.

Estimated Average Glucose

eAG (mg/dL) = 28.7 × A1c − 46.7. A1c 6% ≈ 126; 7% ≈ 154; 8% ≈ 183; 9% ≈ 212; 10% ≈ 240.

C-Peptide & Insulin

C-peptide distinguishes endogenous from exogenous insulin. Low C-peptide with high insulin suggests exogenous insulin (factitious, suicidal); high C-peptide with high insulin suggests insulinoma, sulfonylurea use, or insulin resistance. The 72-hour fast with measurement of glucose, insulin, C-peptide, and proinsulin remains the gold standard for insulinoma diagnosis.

DKA vs HHS

23 Adrenal, Pituitary & Bone Chemistry

Cortisol & the HPA Axis

Aldosterone & Renin

Aldosterone-renin ratio (ARR) screens for primary hyperaldosteronism. ARR >20 with aldosterone >15 ng/dL suggests primary aldosteronism — confirm with saline suppression or oral salt loading. Hold spironolactone 4–6 weeks before testing; other antihypertensives have variable effects and should be adjusted if possible.

Pheochromocytoma

Best initial test is plasma free metanephrines (sensitivity >95%) or 24-hour urine metanephrines. Avoid acetaminophen before plasma sampling (assay interference). False positives from stress, TCAs, MAOIs, clonidine withdrawal, labetalol. Confirm with imaging only after biochemical confirmation.

Pituitary

Calcium Homeostasis & Bone

Bone Turnover Markers

Sex Hormones & Reproductive Testing

24 Urinalysis & Microscopy

Dipstick Components

Microscopy

Spin 10 mL of urine at 2000 rpm for 5 minutes, decant, and resuspend the pellet. Examine under low and high power. Normal findings include occasional hyaline casts, rare RBCs (<2/hpf), rare WBCs (<5/hpf), and squamous epithelial cells (may indicate contamination if numerous).

Proteinuria Quantification

Dipstick is semiquantitative and detects mainly albumin. Quantify with urine protein:creatinine ratio (spot sample approximates 24-hr protein in g/day) or urine albumin:creatinine ratio for microalbuminuria screening. Normal <30 mg/g; microalbuminuria 30–300; macroalbuminuria >300. Nephrotic range proteinuria >3.5 g/day.

25 Gram Stain & Culture Interpretation

Gram Stain Morphology

Blood Culture Interpretation

Standard practice: 2 sets (aerobic + anaerobic, 20 mL each) drawn from separate sites before antibiotics. Time to positivity <12 hours with high-grade organisms suggests true bacteremia; delayed growth at 48–72 hours often represents contamination or low-inoculum organisms. Persistent bacteremia with S. aureus, Candida, or HACEK organisms always warrants endocarditis evaluation.

Urine Culture Thresholds

26 CSF & Body Fluid Analysis

CSF Reference Values

CSF collected in 4 tubes traditionally: tube 1 cell count and chemistries, tube 2 microbiology, tube 3 special studies, tube 4 second cell count (to compare RBC counts for traumatic tap vs SAH). In traumatic tap, RBCs decrease tube 1 to tube 4; in SAH, they remain the same and xanthochromia develops after ~4–12 hours.

Pleural Fluid — Light's Criteria

Exudate if any one of: pleural protein/serum protein >0.5, pleural LDH/serum LDH >0.6, or pleural LDH >2/3 ULN of serum LDH. Transudates are typically bilateral and due to CHF, cirrhosis, or nephrotic syndrome. Exudates suggest infection, malignancy, PE, autoimmune, or pancreatitis. Additional useful tests: pH <7.2 suggests empyema or complicated parapneumonic effusion requiring drainage; glucose <60 in effusion suggests rheumatoid, empyema, TB, or malignancy; amylase ↑ in pancreatitis and esophageal rupture; triglycerides >110 mg/dL in chylothorax.

Ascitic Fluid — SAAG

Serum-ascites albumin gradient = serum albumin − ascites albumin. SAAG ≥1.1 g/dL indicates portal hypertension (cirrhosis, heart failure, Budd-Chiari, massive liver mets); SAAG <1.1 suggests non-portal etiology (peritoneal carcinomatosis, TB peritonitis, pancreatic ascites, nephrotic syndrome). In suspected spontaneous bacterial peritonitis (SBP), PMN count ≥250/μL is diagnostic — treat empirically with 3rd-generation cephalosporin while awaiting culture.

Synovial Fluid

Crystal analysis under polarized light: monosodium urate (gout) = needle-shaped, negatively birefringent, yellow when parallel to axis. Calcium pyrophosphate (pseudogout) = rhomboid, positively birefringent, blue when parallel to axis.

Autoimmune Serology Quick Reference

Tumor Markers

27 Toxicology Screens & Drug Levels

Urine Drug Screen (Immunoassay)

Acetaminophen & Salicylates

Acetaminophen level at 4 hours post-ingestion plotted on the Rumack-Matthew nomogram determines N-acetylcysteine therapy. Treat above the treatment line (>150 μg/mL at 4 hours). For staggered ingestions or unknown timing, treat empirically. Salicylate level: therapeutic 10–30 mg/dL; toxicity >40; severe >80. Mixed respiratory alkalosis (early) + anion gap metabolic acidosis (later) is pathognomonic.

Therapeutic Drug Monitoring

Antiepileptic & Immunosuppressant Monitoring

Phenytoin is heavily protein-bound. In hypoalbuminemia (common in hospitalized patients), the total level underestimates the free (active) drug. Correct: corrected phenytoin = measured / (0.2 × albumin + 0.1). In severe hypoalbuminemia or renal failure, measure free phenytoin directly.

Osmolar Gap & Toxic Alcohols

Calculated osm = 2(Na) + glucose/18 + BUN/2.8 + ethanol/4.6. Osmolar gap = measured − calculated. Normal <10. Elevated in methanol, ethylene glycol, isopropanol, mannitol, propylene glycol, and severe ketoacidosis. Combination of elevated anion gap + elevated osmolar gap + metabolic acidosis suggests toxic alcohol poisoning — treat with fomepizole and consider dialysis.

28 Critical Values, Formulas & Reference

Master Formula Reference

Quick Reference — Expected Compensation

High-Yield Clinical Pearls

Critical Value Callback Checklist

Abbreviations

ABG = arterial blood gas; ACD = anemia of chronic disease; ACTH = adrenocorticotropic hormone; ADAMTS13 = von Willebrand factor-cleaving protease; AFP = alpha-fetoprotein; AG = anion gap; AKI = acute kidney injury; ALP = alkaline phosphatase; ALT = alanine aminotransferase; ANA = antinuclear antibody; ANC = absolute neutrophil count; APS = antiphospholipid syndrome; aPTT = activated partial thromboplastin time; AST = aspartate aminotransferase; ATN = acute tubular necrosis; BHB = beta-hydroxybutyrate; BMP = basic metabolic panel; BNP = B-type natriuretic peptide; BUN = blood urea nitrogen; CBC = complete blood count; CCP = cyclic citrullinated peptide; CK = creatine kinase; CKD = chronic kidney disease; CMP = comprehensive metabolic panel; CRP = C-reactive protein; CSF = cerebrospinal fluid; CTX = C-telopeptide; DIC = disseminated intravascular coagulation; DKA = diabetic ketoacidosis; DOAC = direct oral anticoagulant; EDTA = ethylenediaminetetraacetic acid; eGFR = estimated GFR; FEUrea = fractional excretion of urea; HAGMA = high anion gap metabolic acidosis; HbA1c = glycated hemoglobin; HIT = heparin-induced thrombocytopenia; HHS = hyperosmolar hyperglycemic state; IDA = iron deficiency anemia; INR = international normalized ratio; LDH = lactate dehydrogenase; LFT = liver function test; LMWH = low molecular weight heparin; MAHA = microangiopathic hemolytic anemia; MCH = mean corpuscular hemoglobin; MCHC = mean corpuscular Hb concentration; MCV = mean corpuscular volume; MDS = myelodysplastic syndrome; MELD = Model for End-Stage Liver Disease; NPV = negative predictive value; OGTT = oral glucose tolerance test; PBC = primary biliary cholangitis; PCC = prothrombin complex concentrate; PE = pulmonary embolism; PMN = polymorphonuclear; PPV = positive predictive value; PSC = primary sclerosing cholangitis; PT = prothrombin time; RDW = red cell distribution width; RPI = reticulocyte production index; RTA = renal tubular acidosis; SAAG = serum-ascites albumin gradient; SBP = spontaneous bacterial peritonitis; SIADH = syndrome of inappropriate ADH; SLE = systemic lupus erythematosus; TIBC = total iron-binding capacity; TSH = thyroid stimulating hormone; TT = thrombin time; TTP = thrombotic thrombocytopenic purpura; ULN = upper limit of normal; vWD = von Willebrand disease; vWF = von Willebrand factor.

Final Integration

Laboratory interpretation is ultimately pattern recognition grounded in physiology. The best clinicians do not memorize isolated values; they build internal narratives that explain why a set of numbers makes sense (or doesn't) in a particular patient. When a panel comes back, ask four questions: What is abnormal? Why might this be abnormal? Does the pattern fit a known disease? What is the next test that will confirm or refute my working diagnosis? This discipline — applied to every panel, every time — is what separates clinicians who act on lab results from those who drown in them.

The chapters above work as a reference map, but the most important skill is the habit of returning to first principles whenever a result doesn't fit: verify the specimen, check for confounders, correct for albumin or glucose when applicable, look at the trend not the snapshot, and tie the pattern back to physiology. A clinician who does this consistently will out-perform any diagnostic algorithm, because the clinical question is always richer than the single number. And when the number truly is the answer — a critical potassium, a positive blood culture, a troponin rising on serial draws — the cost of hesitation is measurable in hours of injury. The work of lab interpretation is therefore equal parts skepticism and decisiveness, and both must be practiced deliberately.

Common Interpretive Traps Worth Memorizing