USMLE Lab Values 2026: Essential Reference for Step 1 and Step 2

Complete Blood Count (CBC) — USMLE Reference Values

The complete blood count (CBC) is one of the most commonly ordered and most commonly tested laboratory panels on the USMLE. Knowing normal ranges allows quick identification of abnormalities in vignette-based questions.

Red Blood Cell (RBC) Parameters

Hemoglobin (Hgb): Men 13.5 to 17.5 g/dL; Women 12.0 to 16.0 g/dL. Anemia is defined as Hgb below these thresholds. Hematocrit (Hct): Men 41% to 53%; Women 36% to 46%. Hct is approximately 3× the Hgb value. Red blood cell count (RBC): 4.2 to 5.8 million/μL (men); 3.8 to 5.2 million/μL (women). Mean corpuscular volume (MCV): 80 to 100 fL — used to classify anemia. MCV below 80 = microcytic; MCV 80 to 100 = normocytic; MCV above 100 = macrocytic. Mean corpuscular hemoglobin concentration (MCHC): 31 to 37 g/dL. MCHC below 31 = hypochromic (iron deficiency, thalassemia). Reticulocyte count: 0.5% to 1.5% of red cells — elevated reticulocytes indicate bone marrow response to blood loss or hemolysis. Red cell distribution width (RDW): 11.5% to 14.5% — elevated RDW indicates anisocytosis (variation in RBC size), seen in mixed deficiencies and early iron deficiency.

White Blood Cell (WBC) Parameters

Total WBC: 4,500 to 11,000/μL. Differential counts (approximate normals): Neutrophils 54 to 62% (2,500 to 7,000/μL) — elevated in bacterial infection, stress, steroids; Lymphocytes 25 to 33% (1,000 to 4,000/μL) — elevated in viral infections; Monocytes 3 to 7% — elevated in chronic infections, monocytic leukemia; Eosinophils 1 to 3% — elevated in parasites, allergies, Addison's disease (NAACP mnemonic); Basophils 0 to 1% — elevated in CML; Bands 3 to 5% — elevated bands (left shift) indicate acute bacterial infection or sepsis.

Platelet Parameters

Platelet count: 150,000 to 400,000/μL. Thrombocytopenia: below 150,000 (bleeding risk increases significantly below 50,000; spontaneous bleeding risk below 20,000). Thrombocytosis: above 400,000 (primary thrombocytosis in essential thrombocythemia; reactive in iron deficiency, inflammation, post-splenectomy).

Metabolic Panels — USMLE Reference Values

The basic metabolic panel (BMP) and comprehensive metabolic panel (CMP) are high-yield panels for USMLE — frequently referenced in clinical vignettes involving electrolyte disturbances, renal function, and glucose regulation.

Electrolytes

Sodium (Na+): 136 to 145 mEq/L. Hyponatremia below 136: SIADH (euvolemic), heart failure/cirrhosis (hypervolemic), Addison's disease, hypothyroidism. Hypernatremia above 145: diabetes insipidus, dehydration. Potassium (K+): 3.5 to 5.0 mEq/L. Hypokalemia below 3.5: diuretics, vomiting, Cushing's, hyperaldosteronism. Hyperkalemia above 5.0: renal failure, Addison's, ACE inhibitors, potassium-sparing diuretics, massive tissue destruction. Chloride (Cl-): 98 to 106 mEq/L — used to calculate anion gap. Bicarbonate (HCO3-): 22 to 28 mEq/L. Low bicarbonate in metabolic acidosis; elevated in metabolic alkalosis. Anion gap = Na - (Cl + HCO3) — normal is 8 to 12 mEq/L. Elevated anion gap metabolic acidosis (MUDPILES mnemonic): Methanol, Uremia, Diabetic ketoacidosis, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates.

Renal Function

Blood urea nitrogen (BUN): 7 to 20 mg/dL. Elevated in prerenal azotemia, renal failure, high protein diet, GI bleed. Low in liver failure, malnutrition. Creatinine (Cr): 0.6 to 1.2 mg/dL (men); 0.5 to 1.1 mg/dL (women). Elevated in acute and chronic kidney disease — creatinine doubles when GFR falls by 50%. BUN:Creatinine ratio: normal is approximately 10:1 to 20:1. Ratio above 20:1 suggests prerenal azotemia (dehydration, heart failure). Ratio below 10:1 suggests intrinsic renal disease or low protein intake. Uric acid: Men 3.4 to 7.0 mg/dL; Women 2.4 to 6.0 mg/dL. Elevated in gout, tumor lysis syndrome, Lesch-Nyhan, diuretic use.

Glucose

Fasting glucose: 70 to 100 mg/dL. Prediabetes: 100 to 125 mg/dL (fasting). Diabetes: 126 mg/dL or above (fasting, confirmed on two occasions). Random glucose above 200 mg/dL with symptoms = diabetes. HbA1c: below 5.7% = normal; 5.7 to 6.4% = prediabetes; 6.5% or above = diabetes. Reflects average glucose over approximately 3 months. Target for treated diabetes: below 7% in most patients. Hypoglycemia: below 70 mg/dL — classic Whipple's triad: symptoms + low glucose + symptom resolution with glucose administration.

Cardiac and Liver Markers — USMLE Reference Values

Cardiac and liver enzyme markers are frequently tested in USMLE vignettes — knowing the timing and specificity of each marker is essential for diagnosis questions.

Cardiac Markers

Troponin I and Troponin T: the most sensitive and specific markers for myocardial infarction. Rises within 3 to 4 hours of MI onset; peaks at 24 to 48 hours; returns to normal in 7 to 10 days. High-sensitivity troponin (hsTnI or hsTnT) detects myocardial injury earlier — rises within 1 to 2 hours. Normal troponin values vary by assay — the 99th percentile of a normal reference population is the standard cutoff. Creatine kinase (CK-MB): rises within 3 to 6 hours; peaks at 12 to 24 hours; normalizes within 2 to 3 days. CK-MB is useful for detecting re-infarction (returns to normal faster than troponin, so a second rise is detectable). CK-MB fraction above 5% of total CK is specific for cardiac muscle. Myoglobin: the earliest marker, rises within 1 to 4 hours but lacks specificity (also elevated in skeletal muscle injury). BNP (B-type natriuretic peptide): released from ventricular myocytes in response to volume overload and stretching. BNP above 100 pg/mL is a sensitive marker for heart failure. NT-proBNP (inactive fragment): above 125 pg/mL in patients under 75 is significant; above 450 pg/mL in patients over 75.

Liver Function Tests (LFTs)

AST (aspartate aminotransferase): normal 10 to 40 U/L. Elevated in hepatocellular injury, MI, muscle disease. Not liver-specific. ALT (alanine aminotransferase): normal 7 to 56 U/L. More liver-specific than AST. AST:ALT ratio above 2:1 suggests alcoholic hepatitis (remember: the alcohol bottle has two labels). Alkaline phosphatase (ALP): normal 44 to 147 U/L. Elevated in cholestatic disease (bile duct obstruction, primary biliary cirrhosis), bone disease, pregnancy. GGT (gamma-glutamyl transferase): elevated in hepatic disease and alcohol use — helps distinguish hepatic from bone ALP. Total bilirubin: 0.1 to 1.2 mg/dL. Direct (conjugated) bilirubin above 0.3 mg/dL indicates hepatic or posthepatic cause. Indirect (unconjugated) bilirubin elevation indicates prehepatic cause (hemolysis) or Gilbert syndrome. Albumin: 3.5 to 5.0 g/dL — reduced in chronic liver disease, protein malnutrition, nephrotic syndrome. PT/INR: elevated in liver disease (coagulation factor deficiency), vitamin K deficiency, warfarin use.

Endocrine and Hormone Values — USMLE Reference

Endocrine laboratory values are high-yield for USMLE Step 1 (pathophysiology) and Step 2 CK (diagnosis and management).

Thyroid Function Tests

TSH (thyroid-stimulating hormone): 0.4 to 4.0 mIU/L — the most sensitive test for thyroid dysfunction. Elevated TSH = hypothyroidism (thyroid gland is underactive; pituitary compensates by increasing TSH). Decreased TSH = hyperthyroidism or pituitary failure. Free T4: 0.8 to 1.8 ng/dL. Elevated in hyperthyroidism; decreased in hypothyroidism. Primary hypothyroidism: high TSH + low free T4. Secondary (pituitary) hypothyroidism: low TSH + low free T4. Hyperthyroidism: low TSH + high free T4 (Graves' disease, toxic nodule). Subclinical hypothyroidism: elevated TSH + normal free T4 — treat if TSH above 10 or if symptomatic.

Adrenal Markers

Cortisol (morning, 8 AM): 6 to 23 μg/dL. Low cortisol in Addison's disease, secondary adrenal insufficiency. High cortisol in Cushing's syndrome. ACTH: Low ACTH with low cortisol = primary adrenal insufficiency (Addison's). High ACTH with high cortisol = Cushing's disease (pituitary adenoma) or ectopic ACTH. Low ACTH with high cortisol = adrenal adenoma or exogenous steroid use. 24-hour urine free cortisol: the best screening test for Cushing's syndrome.

Diabetes and Pancreatic Markers

C-peptide: 0.5 to 2.0 ng/mL. Low C-peptide with high insulin = exogenous insulin administration. High C-peptide with high insulin = insulinoma. Absent C-peptide = Type 1 diabetes (autoimmune destruction of beta cells). Insulin: fasting 5 to 20 μU/mL. Amylase: 25 to 125 U/L. Elevated in acute pancreatitis (rises within 2 to 12 hours, returns to normal in 3 to 5 days). Lipase: 0 to 160 U/L. More specific than amylase for pancreatitis; remains elevated longer (up to 7 to 14 days).

USMLE Lab Interpretation Tips

Understanding how to interpret lab values in the context of clinical vignettes — not just knowing normal ranges — is what the USMLE tests. These high-yield interpretation frameworks appear repeatedly across Step 1 and Step 2 CK vignettes.

Acid-Base Interpretation

Arterial blood gas (ABG) normal values: pH 7.35 to 7.45; PaCO2 35 to 45 mmHg; PaO2 75 to 100 mmHg; HCO3 22 to 28 mEq/L. Step-by-step interpretation: Step 1 — Is the pH acidic (below 7.35) or alkaline (above 7.45)? Step 2 — Is the primary disturbance respiratory (PaCO2 abnormal) or metabolic (HCO3 abnormal)? Step 3 — Is there compensation? Appropriate compensation indicates a primary disorder, not a mixed disorder. Step 4 — Calculate the anion gap if metabolic acidosis is present. Respiratory acidosis: high PaCO2, low pH (hypoventilation — COPD, opioid overdose, sleep apnea). Metabolic alkalosis: high HCO3, high pH (vomiting, diuretics, hyperaldosteronism). Metabolic acidosis: low HCO3, low pH (calculate anion gap). Respiratory alkalosis: low PaCO2, high pH (hyperventilation — anxiety, high altitude, PE, early sepsis).

Anemia Classification

Use MCV to classify anemia: Microcytic (MCV below 80) — iron deficiency (most common, low ferritin, high TIBC), thalassemia (low MCV out of proportion to anemia, normal/high ferritin), anemia of chronic disease (usually normocytic, but can be microcytic — low TIBC distinguishes from iron deficiency), lead poisoning. Normocytic (MCV 80 to 100) — anemia of chronic disease (most common normocytic), aplastic anemia, acute blood loss, renal failure (low EPO), hemolytic anemia. Macrocytic (MCV above 100) — vitamin B12 or folate deficiency (megaloblastic, hypersegmented neutrophils), medications (hydroxyurea, methotrexate, 5-FU), alcohol-related, hypothyroidism, liver disease.