USMLE Lab Values: The Complete Reference Guide (+ Memory Tricks)
Lab values are the backbone of clinical reasoning on the USMLE. Every Step 1 and Step 2 CK exam is loaded with questions that hinge on interpreting a basic metabolic panel, a CBC, or an arterial blood gas. If you cannot look at a sodium of 128 mEq/L and immediately recognize hyponatremia, you are going to lose precious seconds on every clinical vignette.
This guide gives you every USMLE lab value you need to know, organized by category, with both conventional and SI units, clinical significance for abnormal results, and memory tricks that actually stick.
Do You Actually Need to Memorize Lab Values?
Here is the good news: the USMLE provides a lab values reference sheet during the exam. You can access it at any time by clicking the "Lab Values" button on the test interface.
Here is the reality: you should still know the major normals by heart. The reference sheet is a crutch, not a strategy. Students who have internalized the high-yield values consistently report faster question completion times and better clinical reasoning under pressure. Every 10 seconds you spend looking up a sodium range is 10 seconds you are not spending on the actual clinical question.
The optimal approach: memorize the high-yield values in this guide (electrolytes, CBC, LFTs, renal function, and ABGs). Use the reference sheet as a safety net for rare values you see less often.
Blood, Plasma, and Serum Values
Electrolytes
These are the values you will use most frequently. Every metabolic panel interpretation starts here.
| Lab Value | Normal Range | SI Units | Memory Trick |
|---|---|---|---|
| Sodium (Na+) | 136–145 mEq/L | 136–145 mmol/L | "Odd numbers 1-3-5", so 136 to 145 |
| Potassium (K+) | 3.5–5.0 mEq/L | 3.5–5.0 mmol/L | "Bananas in a bunch" (a hand has 3.5 to 5) |
| Chloride (Cl−) | 95–105 mEq/L | 95–105 mmol/L | "Follows sodium": subtract ~40 from Na+ |
| Bicarbonate (HCO3−) | 22–28 mEq/L | 22–28 mmol/L | "22 to 28", roughly the age range of med students |
| Calcium (total) | 8.5–10.5 mg/dL | 2.1–2.6 mmol/L | "Gallon of milk" (costs $8.50–$10.50) |
| Calcium (ionized) | 4.5–5.5 mg/dL | 1.1–1.4 mmol/L | "Half of total", roughly half the total range |
| Phosphorus | 2.5–4.5 mg/dL | 0.8–1.5 mmol/L | "Inverse of calcium": when Ca goes up, PO4 goes down |
| Magnesium | 1.5–2.5 mg/dL | 0.75–1.25 mmol/L | "1.5 to 2.5", a simple symmetric range |
Clinical significance:
- High sodium (>145): Dehydration, diabetes insipidus, excess salt. Think water loss.
- Low sodium (<136): SIADH, heart failure, cirrhosis, thiazide diuretics. Most common electrolyte disorder in hospitalized patients.
- High potassium (>5.0): Renal failure, ACE inhibitors, spironolactone, acidosis. ECG: peaked T waves, widened QRS. Life-threatening.
- Low potassium (<3.5): Diuretics, vomiting, diarrhea, alkalosis. ECG: flattened T waves, U waves.
- High calcium (>10.5): "Stones, bones, groans, and psychiatric overtones." Primary hyperparathyroidism, malignancy.
- Low calcium (<8.5): Hypoparathyroidism, vitamin D deficiency, CKD. Chvostek and Trousseau signs.
Renal Function
| Lab Value | Normal Range | SI Units | Memory Trick |
|---|---|---|---|
| BUN | 7–20 mg/dL | 2.5–7.1 mmol/L | "BUN in the oven, 7 to 20" |
| Creatinine | 0.6–1.2 mg/dL | 53–106 µmol/L | "Cr is about 1" since it hovers around 1.0 |
| BUN/Cr ratio | 10–20:1 | — | "Normal is 10–20 to 1" |
| GFR | >90 mL/min/1.73 m² | — | "GFR 90 = A grade" (below 90 is abnormal) |
| Uric acid | 2.5–8.0 mg/dL | 149–476 µmol/L | "Gout at 8" because above 8 raises gout risk |
Clinical significance:
- BUN/Cr >20:1: Prerenal azotemia (dehydration, heart failure). The kidney is underperfused, reabsorbs more urea.
- BUN/Cr <10:1: Intrinsic renal damage, liver disease, malnutrition.
- Elevated creatinine: Chronic kidney disease staging. GFR is a better marker than creatinine alone.
Liver Function Tests
| Lab Value | Normal Range | Memory Trick |
|---|---|---|
| AST (SGOT) | 8–40 U/L | "School hours": school runs 8 to 4(0) |
| ALT (SGPT) | 8–40 U/L | "Same as AST", both 8–40 |
| Alkaline phosphatase | 25–100 U/L | "A quarter to a dollar" (25 to 100) |
| GGT | 0–30 U/L | "GGT for Gin & Tonic", a classic alcohol marker |
| Total bilirubin | 0.1–1.0 mg/dL | "Bili is about 1" |
| Direct bilirubin | 0.0–0.3 mg/dL | "Direct is a third": ~1/3 of total |
| Albumin | 3.5–5.5 g/dL | "Same as potassium range" since both are 3.5–5.5 |
| Total protein | 6.0–7.8 g/dL | "6 to 8, approximately" |
Clinical significance:
- AST > ALT: Alcoholic hepatitis (think "Scotch" = AST). A 2:1 ratio is classic.
- ALT > AST: Viral hepatitis, NAFLD. ALT is more liver-specific.
- Elevated ALP + GGT: Biliary obstruction (cholestatic pattern).
- Elevated direct bilirubin: Obstructive jaundice, Dubin-Johnson, Rotor syndrome.
- Elevated indirect bilirubin: Hemolysis, Gilbert syndrome, Crigler-Najjar.
Lipid Panel
| Lab Value | Optimal | Borderline/High |
|---|---|---|
| Total cholesterol | <200 mg/dL | 200–239 borderline, ≥240 high |
| LDL | <100 mg/dL | ≥160 high; "LDL = Lousy, under 100" |
| HDL | >40 (M), >50 (F) mg/dL | <40 is a risk factor; "HDL = Happy, higher is better" |
| Triglycerides | <150 mg/dL | ≥200 high |
Thyroid Function
| Lab Value | Normal Range | Memory Trick |
|---|---|---|
| TSH | 0.5–5.0 mU/L | "Half to five" |
| Free T4 | 0.7–1.8 ng/dL | "About 1" |
| Free T3 | 2.3–4.2 pg/mL | "2 to 4" |
- High TSH + low T4: Primary hypothyroidism (Hashimoto).
- Low TSH + high T4: Hyperthyroidism (Graves, toxic nodule).
- Low TSH + low T4: Central (secondary) hypothyroidism, suggesting a pituitary problem.
Cardiac Markers
| Lab Value | Normal Range | Memory Trick |
|---|---|---|
| Troponin I | <0.04 ng/mL | "Troponin should be near zero" |
| CK-MB | <5% of total CK | "CK-MB for Myocardial Bad" |
| BNP | <100 pg/mL | "BNP 100 = no heart failure" |
| NT-proBNP | <300 pg/mL | "Triple the BNP cutoff" |
Iron Studies
| Lab Value | Normal Range | Memory Trick |
|---|---|---|
| Serum iron | 60–170 µg/dL | "Iron 60 to 170" |
| TIBC | 250–370 µg/dL | "TIBC 250–370" |
| Ferritin | 12–150 ng/mL (F), 12–300 (M) | "Ferritin = iron stores" |
| Transferrin sat | 20–50% | "Sat 20–50%" |
- Iron deficiency: Low iron, high TIBC, low ferritin, low sat. Most common anemia worldwide.
- Anemia of chronic disease: Low iron, low TIBC, high ferritin. Iron trapped in macrophages.
- Hemochromatosis: High iron, low TIBC, very high ferritin, high sat (>45%).
Hematologic Values
Complete Blood Count
| Lab Value | Male | Female | Memory Trick |
|---|---|---|---|
| WBC | 4,500–11,000/mm³ | 4,500–11,000/mm³ | "4.5 to 11 thousand" |
| RBC | 4.5–5.5 M/mm³ | 4.0–5.0 M/mm³ | "Men slightly higher" |
| Hemoglobin | 13.5–17.5 g/dL | 12.0–16.0 g/dL | "Men 14-ish, women 12-ish" |
| Hematocrit | 38–50% | 36–44% | "Hct ≈ 3× Hgb" |
| Platelets | 150,000–400,000/mm³ | 150,000–400,000/mm³ | "150 to 400 thousand" |
| MCV | 80–100 fL | 80–100 fL | "80–100 = normocytic" |
| RDW | 11.5–14.5% | 11.5–14.5% | "RDW around 13" |
| Reticulocytes | 0.5–1.5% | 0.5–1.5% | "Retics about 1%" |
WBC Differential
| Cell Type | Percentage | Memory Trick |
|---|---|---|
| Neutrophils | 40–70% | "Biggest fraction; Never Let Monkeys Eat Bananas" |
| Lymphocytes | 20–40% | "Number 2" |
| Monocytes | 2–8% | "Mono = 2–8" |
| Eosinophils | 1–4% | "Eos for allergies" |
| Basophils | 0–1% | "Basically zero" |
- Low MCV (<80): Microcytic, including iron deficiency, thalassemia, lead, and sideroblastic. Mnemonic: TAILS.
- High MCV (>100): Macrocytic. Think B12/folate deficiency, alcoholism, liver disease.
- Elevated RDW: Iron deficiency has high RDW; thalassemia trait has normal RDW. Classic differentiator.
Coagulation
| Lab Value | Normal Range | Memory Trick |
|---|---|---|
| PT | 11–15 seconds | "PT for exTrinsic pathway" |
| INR | 0.9–1.1 (2.0–3.0 on warfarin) | "INR 1 normal, 2–3 on warfarin" |
| aPTT | 25–35 seconds | "aPTT for inTrinsic" |
| Fibrinogen | 200–400 mg/dL | "Fibrinogen 200–400" |
| D-dimer | <0.5 µg/mL | "D-dimer rules OUT clot" |
- Elevated PT only: Warfarin, vitamin K deficiency, liver disease (factor VII shortest half-life).
- Elevated aPTT only: Heparin, hemophilia A (VIII) or B (IX), von Willebrand disease.
- Both elevated: DIC, liver failure, massive transfusion.
Cerebrospinal Fluid Values
| Lab Value | Normal Range | Memory Trick |
|---|---|---|
| Opening pressure | 6–20 cm H₂O | "6 to 20" |
| WBC | 0–5 cells/mm³ | "CSF nearly sterile" |
| Glucose | 40–70 mg/dL | "2/3 of serum glucose" |
| Protein | 15–45 mg/dL | "CSF protein 15–45" |
CSF Patterns by Pathology (High-Yield)
| Condition | WBC | Predominant Cell | Glucose | Protein | Pressure |
|---|---|---|---|---|---|
| Bacterial meningitis | Very high (>1000) | Neutrophils | Low | High | High |
| Viral meningitis | Moderate (10–500) | Lymphocytes | Normal | Normal/slight ↑ | Normal |
| TB meningitis | Moderate (50–500) | Lymphocytes | Low | High | High |
| Fungal meningitis | Moderate (10–500) | Lymphocytes | Low | High | High |
| Guillain-Barré | Normal | — | Normal | High (albuminocytologic dissociation) | Normal |
| Subarachnoid hemorrhage | RBCs | RBCs | Normal | High | High |
Key distinction: Bacterial meningitis is the only common cause of CSF neutrophilia with low glucose. Neutrophils + low glucose = bacteria until proven otherwise.
Arterial Blood Gas Values
| Lab Value | Normal Range | Memory Trick |
|---|---|---|
| pH | 7.35–7.45 | "7.4 is the magic number" |
| PaCO₂ | 35–45 mmHg | "CO₂ is 40 ± 5" |
| PaO₂ | 80–100 mmHg | "O₂ should be 80–100" |
| HCO₃⁻ | 22–28 mEq/L | "Bicarb 22–28" |
| O₂ saturation | 95–100% | "Sat above 95" |
ABG Interpretation Framework
- Look at pH. Acidic (<7.35) or alkalotic (>7.45)?
- Identify the primary disorder. Does CO₂ explain the pH (respiratory) or HCO₃⁻ (metabolic)?
- Check for compensation. Is the other system compensating appropriately?
- For metabolic acidosis, calculate the anion gap. AG = Na − (Cl + HCO₃). Normal = 8–12.
| Disorder | pH | PaCO₂ | HCO₃⁻ | Compensation |
|---|---|---|---|---|
| Metabolic acidosis | Low | Low | Low | Winter's: PaCO₂ = 1.5(HCO₃) + 8 ± 2 |
| Metabolic alkalosis | High | High | High | PaCO₂ rises 0.7 per 1 mEq HCO₃ rise |
| Resp acidosis (acute) | Low | High | Normal | HCO₃ rises 1 per 10 mmHg CO₂ |
| Resp acidosis (chronic) | Low/normal | High | High | HCO₃ rises 3.5 per 10 mmHg CO₂ |
| Resp alkalosis (acute) | High | Low | Normal | HCO₃ falls 2 per 10 mmHg CO₂ |
| Resp alkalosis (chronic) | High/normal | Low | Low | HCO₃ falls 5 per 10 mmHg CO₂ |
MUDPILES for elevated anion gap metabolic acidosis: Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates.
High-Yield Lab Patterns for Clinical Vignettes
The USMLE rarely asks you to recall a normal range in isolation. It tests constellation patterns:
- Prerenal azotemia: BUN/Cr >20:1, low urine sodium (<20), high urine osmolality, elevated BUN.
- DKA: High anion gap metabolic acidosis, glucose >250, positive ketones, low bicarb.
- Hepatocellular injury: AST/ALT in thousands, AST/ALT <1 (viral) or >2 (alcoholic), elevated bilirubin.
- Nephrotic syndrome: Proteinuria >3.5 g/day, hypoalbuminemia, hyperlipidemia, lipiduria.
- DIC: Elevated PT, elevated aPTT, low fibrinogen, elevated D-dimer, low platelets, schistocytes.
- Iron deficiency vs. thalassemia: Both microcytic. Iron deficiency: high RDW, low ferritin, high TIBC. Thalassemia: normal RDW, normal ferritin.
QuantaPrep's adaptive learning engine identifies which lab patterns give you the most trouble and adjusts your question mix accordingly. The AI tutor explains not just why an answer is correct, but walks you through the lab interpretation step by step.
Frequently Asked Questions
Are USMLE lab values given during the exam?
Yes. The USMLE provides a laboratory reference sheet accessible throughout the exam via the "Lab Values" button. However, experienced test-takers recommend memorizing high-yield values (electrolytes, CBC, LFTs, renal panel, ABGs) to save time. The reference sheet works best as a backup for uncommon values.
Do Step 1 and Step 2 CK use the same lab values?
Yes. The USMLE uses the same standardized reference ranges across all Step exams. Step 1 focuses on pathophysiology (why is this value abnormal?), while Step 2 CK focuses on clinical management (what do you do about it?).
What units does the USMLE use?
The USMLE primarily uses conventional (US standard) units. The reference sheet includes SI units, and some questions may present values in SI. Prioritize conventional units for memorization.
What are the most tested lab values?
Based on question analysis: sodium/potassium (electrolyte disorders), hemoglobin/MCV (anemia classification), AST/ALT (liver pathology), creatinine/BUN (renal function), arterial blood gases (acid-base disorders), and CSF analysis (meningitis differentiation).
What is the anion gap and why does it matter?
The anion gap (Na − Cl − HCO₃, normal 8–12 mEq/L) classifies metabolic acidosis. Elevated AG means an unmeasured acid is present (MUDPILES). Normal AG with metabolic acidosis usually means bicarbonate loss (diarrhea, RTA). This distinction is tested heavily on both Step 1 and Step 2 CK.
How should I study lab values effectively?
Learn values in clinical context, not isolation. Use spaced repetition. Do practice questions early, because you learn more from getting a question wrong and reading the explanation than re-reading a table. Focus on ratios and relationships (BUN/Cr, AST/ALT, anion gap). QuantaPrep gives you unlimited USMLE-style questions to practice interpreting labs in clinical vignettes, free with no credit card required.
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