Table of Contents
Overview
Common laboratory values are heavily tested on the USMLE because they guide diagnosis and management. At this level your goal is not to memorize every possible value, but to recognize normal reference ranges, understand which way a value moves in common conditions, and quickly identify life threatening abnormalities.
For the USMLE, you are usually given a reference range in the question stem or in a table at the beginning of the exam. You must still know what is clearly high, low, or critical, and how basic groups of tests relate to each other.
This chapter focuses on high yield lab panels and key individual values. Detailed physiology, pathophysiology, and statistics are covered in other chapters.
On the USMLE, never ignore:
• Markedly abnormal electrolytes
• Very low hemoglobin or platelets
• Extreme glucose abnormalities
• Elevated cardiac markers or troponin
• High INR in a bleeding patient
These often indicate emergencies and drive the correct answer.
Complete Blood Count
The complete blood count (CBC) evaluates red blood cells, white blood cells, and platelets. For test purposes, approximate adult ranges are usually sufficient. Values differ by lab, sex, and age, but the pattern of abnormality is what matters.
Red Blood Cell Indices
Red cell values help classify anemia and polycythemia.
| Test | Typical Adult Range | High Suggests | Low Suggests |
|---|---|---|---|
| Hemoglobin (Hgb) | Men ~13.5–17.5 g/dL, Women ~12–15.5 g/dL | Polycythemia, chronic hypoxia | Anemia, acute blood loss |
| Hematocrit (Hct) | Men ~41–53%, Women ~36–46% | Dehydration, polycythemia | Anemia, overhydration |
| RBC count | Men ~4.7–6.1 M/µL, Women ~4.2–5.4 M/µL | Polycythemia | Anemia |
The mean corpuscular volume (MCV) classifies anemia by size.
| Index | Normal Range | Interpretation if Abnormal |
|---|---|---|
| MCV | ~80–100 fL | $<80$ fL microcytic, $>100$ fL macrocytic |
| MCHC | ~32–36 g/dL | Low in hypochromic states |
| RDW | ~11.5–14.5% | High RDW means variable cell sizes |
For USMLE questions, expect to use MCV and RDW to distinguish iron deficiency from thalassemia, or megaloblastic from nonmegaloblastic macrocytosis, not to recall exact numeric cutoffs.
White Blood Cells
The total white blood cell (WBC) count and differential help distinguish infection types, leukemias, and immune disorders.
| Test | Typical Range | High Suggests | Low Suggests |
|---|---|---|---|
| WBC count | ~4,000–11,000 /µL | Infection, inflammation, leukemia | Bone marrow failure, severe infection, drugs |
Differential patterns are more important than exact percentages.
| Cell Type | Typical Proportion | High Suggests (USMLE focus) |
|---|---|---|
| Neutrophils | ~40–70% | Bacterial infection, stress, steroids |
| Lymphocytes | ~20–40% | Viral infection, some leukemias |
| Monocytes | ~2–8% | Chronic inflammation, TB, recovery phase |
| Eosinophils | ~1–4% | Allergies, asthma, parasites |
| Basophils | <1% | Myeloproliferative disease, allergy |
Bands and left shift patterns are often mentioned qualitatively rather than with explicit numbers.
Platelets
Platelets are central to bleeding and clotting questions.
| Test | Typical Range | High Suggests | Low Suggests |
|---|---|---|---|
| Platelet count | ~150,000–400,000 /µL | Reactive thrombocytosis, myeloproliferative disease | Thrombocytopenia, risk of bleeding |
• Platelets <50,000 /µL increase bleeding risk.
• Platelets <20,000 /µL are associated with spontaneous serious bleeding.
On USMLE, severe thrombocytopenia in a bleeding patient often requires platelet transfusion.
Basic Metabolic Panel and Electrolytes
The basic metabolic panel (BMP) includes major electrolytes, kidney function markers, and glucose. On USMLE, changes in direction and relative patterns matter more than exact numbers.
Sodium, Potassium, and Chloride
| Electrolyte | Typical Range | Key Patterns |
|---|---|---|
| Sodium (Na⁺) | ~135–145 mEq/L | Hyponatremia with confusion, seizures; hypernatremia with dehydration, neurologic signs |
| Potassium (K⁺) | ~3.5–5.0 mEq/L | Hyperkalemia with peaked T waves, arrhythmias; hypokalemia with U waves, arrhythmias, muscle weakness |
| Chloride (Cl⁻) | ~98–106 mEq/L | Moves with sodium and acid base changes |
On exams, abnormal potassium is especially critical because of its effect on the heart.
Critical K⁺ levels:
• K⁺ <3.0 mEq/L or >6.0 mEq/L can cause dangerous arrhythmias.
Look for ECG changes and immediate treatment in answer choices.
Bicarbonate and Anion Gap
Serum bicarbonate (often reported as CO₂ content) reflects metabolic acid base status.
| Test | Typical Range |
|---|---|
| Bicarbonate (HCO₃⁻) | ~22–28 mEq/L |
The anion gap helps classify metabolic acidosis.
$$\text{Anion gap} = \text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-)$$
Typical anion gap is about 8–12 mEq/L. On the USMLE, you are often told whether it is high or normal. You must interpret high anion gap acidosis with conditions like lactic acidosis or ketoacidosis, but detailed causes are discussed elsewhere.
BUN and Creatinine
Blood urea nitrogen (BUN) and creatinine assess kidney function.
| Test | Typical Range | High Suggests |
|---|---|---|
| BUN | ~7–20 mg/dL | Renal failure, increased protein catabolism, hypovolemia |
| Creatinine | ~0.6–1.3 mg/dL | Reduced GFR, kidney injury |
The BUN to creatinine ratio pattern is more testable than the exact value.
A rising creatinine, especially with decreased urine output, indicates acute kidney injury. Always consider nephrotoxic drugs or volume status in the stem.
Glucose
| Test | Typical Range (Fasting) | USMLE Focus |
|---|---|---|
| Serum glucose | ~70–99 mg/dL | Diagnosis and management of diabetes, hypoglycemia |
You must recognize markedly low or high values and their acute consequences.
• Severe hypoglycemia usually <55 mg/dL, especially with symptoms.
• Diabetic emergencies often have glucose >250 mg/dL and other criteria.
Do not confuse units. In USMLE stems values are in mg/dL unless stated otherwise.
Comprehensive Metabolic Panel and Liver Tests
The comprehensive metabolic panel (CMP) includes the BMP plus liver function tests and some proteins. The USMLE asks you to interpret patterns that suggest hepatocellular injury versus cholestasis or synthetic failure.
Liver Enzymes
| Test | Typical Range | Pattern |
|---|---|---|
| AST | ~10–40 U/L | Elevation suggests liver, muscle, or other tissue damage |
| ALT | ~7–56 U/L | More specific for liver injury |
| Alkaline phosphatase (ALP) | ~44–147 U/L | Cholestasis, bone disease, pregnancy |
The exact numeric upper limit varies, but a several fold increase is usually presented in questions. Patterns of AST and ALT relative to ALP, and their ratios, are more important than the precise upper normal.
Bilirubin and Synthetic Function
| Test | Typical Range | High or Low Suggests |
|---|---|---|
| Total bilirubin | ~0.3–1.2 mg/dL | High with jaundice, hemolysis, cholestasis |
| Albumin | ~3.5–5.0 g/dL | Low in chronic liver disease, nephrotic syndrome, malnutrition |
| Prothrombin time (PT)/INR | See below | Prolonged in liver failure, vitamin K deficiency, warfarin use |
Albumin and INR together are high yield markers of liver synthetic capacity. Chronic liver failure often presents with low albumin and prolonged INR.
Arterial Blood Gases
Arterial blood gases (ABG) identify acid base disorders and oxygenation status. For USMLE questions you rarely have to remember precise normal values, but you must recognize the direction of change.
Typical reference:
| Parameter | Approximate Normal |
|---|---|
| pH | 7.35–7.45 |
| PaCO₂ | 35–45 mm Hg |
| HCO₃⁻ | 22–28 mEq/L |
| PaO₂ | 80–100 mm Hg |
Use pH, PaCO₂, and HCO₃⁻ to classify the primary disorder and compensation, explained in detail in acid base physiology. For this chapter, remember that pH and PaCO₂ move in opposite directions in primary respiratory disorders and in the same direction in primary metabolic disorders when compensation occurs.
A pH <7.2 or >7.6 is life threatening. Severe acidosis or alkalosis often implies urgent management, even before complete diagnostic clarification.
Coagulation Studies
Coagulation tests help evaluate bleeding or clotting tendency. On USMLE they often signal drug effects or specific factor deficiencies.
| Test | What It Measures | Typical Range | Prolongation Suggests |
|---|---|---|---|
| PT | Extrinsic and common pathway | ~11–15 s | Warfarin effect, vitamin K deficiency, liver disease, factor VII deficiency |
| INR | Standardized PT | ~0.8–1.2 | Therapeutic range usually 2–3 on warfarin |
| aPTT | Intrinsic and common pathway | ~25–35 s | Heparin effect, hemophilia, factor deficiencies, lupus anticoagulant |
| Bleeding time (historical) | Platelet function | Variable | Platelet dysfunction, von Willebrand disease |
• A very high INR, especially >4 in a patient on warfarin, greatly increases bleeding risk.
• Prolonged aPTT with normal PT suggests intrinsic pathway problems, such as hemophilia.
• Prolonged PT and aPTT together often indicate severe liver disease or disseminated intravascular coagulation.
Cardiac and Muscle Markers
Cardiac markers help diagnose myocardial infarction and muscle injury.
| Marker | Typical Use | Key Point |
|---|---|---|
| Troponin I/T | Most specific for myocardial damage | Rises within hours, stays elevated for days |
| CK-MB | Cardiac muscle, less specific than troponin | Can help detect reinfarction over shorter time span |
| Total CK | General muscle injury | Elevated in rhabdomyolysis, muscular disease |
You will not be asked to recall the numeric upper limit, but you must recognize that any significant elevation above the reference range is concerning for tissue damage.
Lipid Panel
The lipid panel is important for cardiovascular risk and some genetic conditions.
| Test | Approximate Desirable Level |
|---|---|
| Total cholesterol | <200 mg/dL |
| LDL cholesterol | <100 mg/dL (lower in high risk patients) |
| HDL cholesterol | >40 mg/dL (men), >50 mg/dL (women) |
| Triglycerides | <150 mg/dL |
For the exam, relative changes and risk stratification are more important than exact cutoffs. Very high triglycerides are associated with pancreatitis risk.
Urinalysis
Urinalysis provides rapid insight into kidney function, infection, and systemic disease. Most urinalysis results are given as positive or negative, or graded as trace, 1+, 2+, etc., rather than exact numbers.
Common components:
| Component | Normal Finding | USMLE Relevance |
|---|---|---|
| Appearance | Clear, yellow | Cloudy or foul suggests infection |
| Specific gravity | ~1.002–1.030 | Concentrating ability, hydration status |
| pH | ~4.5–8.0 | Acid base effects, certain stone types |
| Protein | Negative or trace | Proteinuria suggests glomerular disease |
| Glucose | Negative | Positive in uncontrolled diabetes |
| Ketones | Negative | Positive in ketoacidosis, starvation |
| Blood | Negative | Hematuria or myoglobinuria |
| Leukocyte esterase, nitrites | Negative | Positive suggests infection |
Microscopic examination (RBC casts, WBC casts, granular casts, crystals) is highly testable, but the presence or absence is more important than their actual numeric count.
Endocrine and Hormonal Tests
Endocrine tests often appear as isolated values or patterns rather than full panels. You usually receive reference ranges in the stem, but you should know whether a value is obviously high or low and how it fits into feedback loops.
Common values:
| Test | Typical Adult Range | Used For |
|---|---|---|
| TSH | ~0.4–4.0 µIU/mL | Primary thyroid disorders |
| Free T₄ | ~0.8–1.8 ng/dL | Thyroid hormone status |
| Cortisol (morning) | ~5–25 µg/dL | Adrenal function |
| Prolactin | Variable, often <20 ng/mL | Pituitary disorders |
| Fasting insulin, C peptide | Lab specific | Distinguish endogenous vs exogenous insulin |
On USMLE, trends and combinations such as high TSH with low T₄ or the reverse are more important than memorizing the exact numbers.
Hematology and Iron Studies
Iron studies help differentiate causes of anemia.
| Test | Typical Range | High Suggests | Low Suggests |
|---|---|---|---|
| Serum iron | ~60–170 µg/dL | Hemolysis, iron overload | Iron deficiency |
| Ferritin | ~12–300 ng/mL (men), ~12–150 ng/mL (women) | Iron overload, inflammation | Iron deficiency |
| TIBC | ~240–450 µg/dL | Iron deficiency | Anemia of chronic disease |
| Transferrin saturation | ~20–50% | Hemochromatosis | Iron deficiency |
These values are often summarized in tables on the exam, and patterns, not precise cutoffs, are tested.
Miscellaneous High Yield Values
Certain lab values recur frequently and are helpful to recognize even approximately.
| Test | Typical Range | USMLE Point |
|---|---|---|
| Serum calcium (total) | ~8.5–10.5 mg/dL | Hypocalcemia with tetany, hypercalcemia with stones, bones, groans, psychiatric overtones |
| Ionized calcium | ~4.5–5.6 mg/dL | Affected by albumin less than total calcium |
| Serum magnesium | ~1.5–2.5 mEq/L | Low with torsades risk, high with loss of reflexes |
| Phosphate | ~2.5–4.5 mg/dL | Moves opposite to calcium in many conditions |
| Lactate | Lab specific, often <2 mmol/L | Elevated in shock, hypoxia |
| ESR, CRP | Variable | Inflammation markers, non specific but high yield in many diseases |
For many of these, the exam will give you a normal value, but you should recognize when a number is clearly abnormal and when severity implies emergency care.
When you see obviously extreme values in any lab test in a question, assume the test writer wants you to react clinically. Very abnormal numbers usually mean the best next step is urgent stabilization before further workup.
Using Lab Values Efficiently on the USMLE
During the exam, time pressure makes it hard to inspect long lab tables. To remain efficient:
Focus first on vital labs that imply immediate danger. Scan for potassium, sodium, glucose, hemoglobin, platelets, pH, and creatinine. Then, look for pattern forming clusters that fit the clinical picture, such as liver enzymes with bilirubin, or BUN and creatinine together.
Pay close attention to the reference ranges printed in the stem or provided in the exam booklet. Exact ranges vary, and for border zone values the exam expects you to use the given range, not your memory.
Finally, review common panels and their typical ranges repeatedly. Over time, you will not actively think about the numbers, but you will instantly recognize patterns like “microcytic anemia,” “cholestatic liver injury,” or “high anion gap metabolic acidosis,” which is the real goal for test day.