USMLE Lab Values: The Complete Reference Guide (+ Memory Tricks)
A 54-year-old woman presents with fatigue and confusion. Her serum sodium is 128 mEq/L. You have about 90 seconds per question. If you need to look up whether 128 is low, you have already lost time you cannot recover.
Lab value interpretation underpins a significant fraction of USMLE questions across both Step 1 and Step 2 CK — from electrolyte disorders to ABG analysis to CSF differential diagnosis. This reference compiles every high-yield lab value organized by category, with conventional and SI units, clinical significance of abnormal results, and retention aids designed to reduce lookup dependency on exam day.
Do You Actually Need to Memorize Lab Values?
The USMLE provides a lab values reference sheet accessible via the "Lab Values" button throughout the exam, but it is a safety net, not a strategy. Memorize the high-yield values (electrolytes, CBC, LFTs, renal function, ABGs) and use the reference sheet only for rare values. Every 10 seconds spent looking up a sodium range is 10 seconds not spent on clinical reasoning.
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 clinical vignettes integrate lab value interpretation into every relevant question, so you practice reading panels in context rather than memorizing ranges in isolation.
Ten Lab Value Traps That Appear on Every Exam
Beyond knowing the normal ranges, these are the interpretation traps that USMLE question writers use repeatedly. Each exploits a common reasoning error that students make even when they know the reference values.
1. "Normal" potassium in acidosis is actually low. Acidosis shifts potassium extracellularly, artificially inflating the serum measurement. A K+ of 4.0 mEq/L in a patient with DKA actually represents total body potassium depletion — once acidosis is corrected, K+ will drop dangerously. The clinical rule: treat the hypokalemia before correcting the acidosis aggressively. Questions presenting a "normal" K+ in an acidotic patient are testing whether you recognize the hidden deficit.
2. Low albumin makes total calcium misleadingly low. Correct calcium for albumin: for each 1 g/dL albumin below 4.0, add 0.8 mg/dL to the measured calcium. A total Ca of 7.8 mg/dL with albumin of 2.0 g/dL = corrected Ca of 9.4 mg/dL (normal). Without this correction, you would diagnose hypocalcemia that does not exist. This correction formula appears in some form on nearly every Step 1 exam.
3. BUN/creatinine ratio distinguishes prerenal from intrinsic renal failure. Ratio >20:1 = prerenal (dehydration, heart failure — the kidney is underperfused and reabsorbs more urea). Ratio <15:1 = intrinsic renal damage (the tubules are damaged and cannot concentrate urea). This ratio is tested almost every exam cycle and is the first step in evaluating any patient with elevated creatinine.
4. Elevated anion gap with normal lactate and ketones = think toxins. Methanol, ethylene glycol, and salicylate poisoning all cause elevated anion gap metabolic acidosis. If the common causes (lactic acidosis, DKA) are ruled out in the question stem, the question is almost certainly testing a toxicology scenario. The osmolar gap (measured osmolality minus calculated osmolality) further distinguishes methanol/ethylene glycol from salicylates.
5. Iron studies in anemia of chronic disease vs. iron deficiency. Both have low serum iron. The distinguishing test is ferritin: LOW in iron deficiency, NORMAL or HIGH in anemia of chronic disease (ferritin is an acute phase reactant that rises with inflammation). TIBC is high in iron deficiency (the body makes more transferrin to scavenge scarce iron), low in chronic disease. This distinction is tested in some form on virtually every Step 1 exam.
6. Troponin elevation does not equal MI. Troponin rises in myocarditis, PE, renal failure, sepsis, and heart failure. The question stem must support acute coronary syndrome (chest pain, ECG changes, risk factors) before you can diagnose MI. A question presenting troponin elevation in a patient with recent sepsis or PE is testing whether you jump to "MI" reflexively or consider the differential.
7. Lipase is more specific than amylase for pancreatitis. Amylase also rises in parotitis, bowel obstruction, ectopic pregnancy, and macroamylasemia. If both lipase and amylase are answer options for confirming pancreatitis, choose lipase. Lipase is also elevated longer (up to 14 days vs. 3-5 days for amylase), making it more useful for late presentations.
8. TSH and free T4 — always interpret together. Low TSH + high free T4 = hyperthyroid. High TSH + low free T4 = hypothyroid. The trap: High TSH + normal free T4 = subclinical hypothyroid, and low TSH + normal free T4 = subclinical hyperthyroid. The exam frequently tests the subclinical states because students who only learned the classic patterns miss them.
9. D-dimer is sensitive but not specific. A negative D-dimer rules OUT PE/DVT in low-probability patients (high negative predictive value). A positive D-dimer rules out NOTHING — it is elevated in infection, cancer, pregnancy, surgery, recent trauma, and almost any inflammatory state. A question presenting a positive D-dimer is not asking you to diagnose PE; it is asking you what to do next (typically CT pulmonary angiography).
10. ABG vs. VBG — know when each is required. Venous blood gas (VBG) pH is typically within 0.03-0.05 of arterial blood gas (ABG) pH. Questions that specifically ask about oxygenation (PaO2, A-a gradient) require ABG because venous blood has already delivered its oxygen. Questions about acid-base status alone can use VBG. If a question asks you to calculate the A-a gradient and only provides VBG values, that is insufficient data — and recognizing that distinction may be the point of the question.
Lab Value Interpretation: Common 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). Test your lab value interpretation with clinical vignettes — create a free account at quantaprep.com.
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