Table of Contents
Introduction
Normal laboratory values are reference ranges that help you interpret test results on the USMLE. For beginners, the goal is not to memorize every possible number, but to become familiar with the core values that appear repeatedly in questions. Later chapters will discuss how to use these values in diagnosis and management. Here, the focus is simply on what the common normal ranges look like and how they are typically reported.
Always remember that exact reference ranges can vary slightly between laboratories. USMLE questions usually use standard textbook ranges, and when a precise cutoff matters, the question stem will often specify it.
Normal values are guides, not absolute truths. Mild deviations can be normal for some patients. Interpret numbers only in clinical context.
Units and Conventions
On USMLE-style questions you will see values in both conventional and SI units. You must recognize both formats. For example, serum sodium can be written as 140 mEq/L or 140 mmol/L, which clinically are equivalent in common use.
Many chemistry values and blood gas values are given with one significant decimal place, while cell counts are often given with scientific notation or powers of ten. Be comfortable recognizing patterns, such as a white blood cell count of $7 \times 10^3/\mu L$ being normal.
Hematology Values
These values describe cells in the blood and basic indices of anemia or polycythemia. You only need approximate normal ranges for exam purposes.
| Test | Normal Range (Adult) | Notes |
|---|---|---|
| Hemoglobin, male | 13.5 to 17.5 g/dL | Approximate |
| Hemoglobin, female | 12.0 to 16.0 g/dL | Approximate |
| Hematocrit, male | 41 to 53 % | |
| Hematocrit, female | 36 to 46 % | |
| RBC count, male | 4.7 to 6.1 million/µL | |
| RBC count, female | 4.2 to 5.4 million/µL | |
| MCV | 80 to 100 fL | Microcytic < 80, macrocytic > 100 |
| MCHC | 32 to 36 g/dL | |
| WBC count | 4,000 to 11,000/µL | $4\text{ to }11 \times 10^3/\mu L$ |
| Platelets | 150,000 to 400,000/µL | $150\text{ to }400 \times 10^3/\mu L$ |
| Reticulocyte count | 0.5 to 2.0 % | |
| ESR, male | 0 to 15 mm/hr | Varies with age |
| ESR, female | 0 to 20 mm/hr | Varies with age |
On USMLE, hemoglobin < 13 g/dL in men or < 12 g/dL in women is usually considered anemia. Platelets < 150,000/µL suggest thrombocytopenia.
White Blood Cell Differential
Beyond total WBC count, questions often include relative percentages of different leukocytes. Know the typical pattern, not the exact decimals.
| Cell Type | Approximate Relative Percentage |
|---|---|
| Neutrophils | 40 to 60 % |
| Lymphocytes | 20 to 40 % |
| Monocytes | 2 to 8 % |
| Eosinophils | 1 to 4 % |
| Basophils | 0.5 to 1 % |
| Band forms | 0 to 5 % |
An increase in band forms is often described qualitatively as a left shift. You do not need a strict numeric cutoff for the exam.
Basic Metabolic Panel
These core electrolytes and metabolites appear constantly in Step questions. Learn these normal ranges very well and recognize their patterns.
| Test | Normal Range |
|---|---|
| Sodium (Na⁺) | 135 to 145 mEq/L |
| Potassium (K⁺) | 3.5 to 5.0 mEq/L |
| Chloride (Cl⁻) | 98 to 106 mEq/L |
| Bicarbonate (HCO₃⁻) | 22 to 28 mEq/L |
| Blood urea nitrogen | 7 to 20 mg/dL |
| Creatinine, male | 0.6 to 1.3 mg/dL |
| Creatinine, female | 0.5 to 1.1 mg/dL |
| Glucose, fasting | 70 to 99 mg/dL |
| Calcium, total | 8.5 to 10.5 mg/dL |
| Magnesium | 1.5 to 2.5 mEq/L |
| Phosphate | 2.5 to 4.5 mg/dL |
A normal anion gap is roughly 8 to 12 mEq/L when calculated as Na⁺ − (Cl⁻ + HCO₃⁻).
Liver Function and Hepatobiliary Tests
These tests reflect hepatocellular injury, biliary obstruction, and protein synthetic function. On the USMLE, patterns matter more than exact numbers, but knowing typical upper limits helps.
| Test | Typical Normal Range |
|---|---|
| AST | 10 to 40 U/L |
| ALT | 7 to 56 U/L |
| Alkaline phosphatase | 40 to 120 U/L |
| Total bilirubin | 0.3 to 1.2 mg/dL |
| Direct (conjugated) bilirubin | 0 to 0.3 mg/dL |
| Albumin | 3.5 to 5.0 g/dL |
| Total protein | 6.0 to 8.0 g/dL |
| GGT | 0 to 30 U/L (lab dependent) |
Serum albumin < 3.5 g/dL usually indicates impaired synthetic function or protein loss. Elevated direct bilirubin suggests cholestasis or impaired excretion.
Lipid Panel
You should know the standard adult targets, particularly for cardiovascular risk questions. Absolute normal values can vary, but board questions usually use these benchmarks.
| Test | Optimal / Desirable Value |
|---|---|
| Total cholesterol | < 200 mg/dL |
| LDL cholesterol | < 100 mg/dL (general target) |
| HDL cholesterol, male | > 40 mg/dL |
| HDL cholesterol, female | > 50 mg/dL |
| Triglycerides | < 150 mg/dL |
USMLE questions often refer to these as cutoffs for risk stratification and treatment decisions, even if real world targets can differ based on comorbidities.
Coagulation Studies
Clotting tests are frequently tested in pharmacology and hematology questions. Focus on typical times and therapeutic targets for anticoagulation.
| Test | Normal Range |
|---|---|
| PT | 11 to 15 seconds |
| INR | 0.8 to 1.2 |
| aPTT | 25 to 40 seconds |
| Bleeding time | 2 to 7 minutes (lab and method vary) |
| Fibrinogen | 200 to 400 mg/dL |
For most warfarin therapy indications, the therapeutic INR is about 2.0 to 3.0. For unfractionated heparin, the target aPTT is usually about 1.5 to 2.5 times normal.
Arterial Blood Gases
Blood gas interpretation is a high yield skill. You must remember the normal arterial values and how they cluster.
| Parameter | Normal Arterial Value |
|---|---|
| pH | 7.35 to 7.45 |
| PaCO₂ | 35 to 45 mm Hg |
| PaO₂ | 80 to 100 mm Hg |
| HCO₃⁻ (calculated) | 22 to 26 mEq/L |
| O₂ saturation | 95 to 100 % |
For ABG questions, think: pH around 7.4, PaCO₂ around 40 mm Hg, HCO₃⁻ around 24 mEq/L.
Renal Function and Urinalysis
Renal values overlap with the basic metabolic panel but there are additional tests that appear often. Only core values are needed for an introductory view.
| Test | Typical Normal Value |
|---|---|
| Serum creatinine, male | 0.6 to 1.3 mg/dL |
| Serum creatinine, female | 0.5 to 1.1 mg/dL |
| BUN | 7 to 20 mg/dL |
| BUN/Cr ratio | About 10 to 20 |
| Urine specific gravity | 1.002 to 1.030 |
| Urine pH | 4.5 to 8.0 |
| Daily urine protein excretion | < 150 mg/day |
| Microalbumin (screening) | < 30 mg/day |
Estimated GFR formulas will be discussed in the clinical calculators chapter, not here.
Endocrine Laboratory Values
Endocrine questions frequently show hormone levels. Normal ranges can be wide and lab specific, but these approximate ranges give you a useful reference for the exam.
| Test | Approximate Normal Range (Adult) |
|---|---|
| TSH | 0.4 to 4.0 µU/mL |
| Free T₄ | 0.8 to 1.8 ng/dL |
| Total T₄ | 4.5 to 11.2 µg/dL |
| Fasting plasma glucose | 70 to 99 mg/dL |
| 2 hr OGTT (non diabetic) | < 140 mg/dL |
| HbA1c | < 5.7 % (non diabetic) |
| Serum cortisol (8 AM) | 5 to 25 µg/dL |
| Serum prolactin, female | < 25 ng/mL |
| Serum prolactin, male | < 20 ng/mL |
Diagnostic cutoffs for diabetes and specific endocrine syndromes are handled in the relevant disease chapters.
Cardiac Markers
Acute coronary syndrome questions often involve trends in cardiac biomarkers, but you should still know the usual upper reference limits.
| Marker | Typical Upper Reference Limit |
|---|---|
| Troponin I | < 0.04 ng/mL |
| Troponin T | < 0.01 to 0.03 ng/mL |
| CK total | 20 to 200 U/L |
| CK-MB fraction | < 5 to 6 % of total CK |
| BNP | < 100 pg/mL |
Actual lab cutoffs can vary. On USMLE, question stems will often provide the relevant threshold.
Pancreatic and Gastrointestinal Enzymes
These values appear in questions on pancreatitis, malabsorption, and biliary disease.
| Test | Typical Normal Range |
|---|---|
| Amylase | 25 to 125 U/L |
| Lipase | 10 to 140 U/L |
| Serum gastrin | < 100 pg/mL |
Again, the diagnostic criteria for diseases that use these values are covered separately.
Infection and Inflammation Markers
Inflammatory markers are nonspecific, but they are important in systemic disease and sepsis questions.
| Test | Normal Range |
|---|---|
| C reactive protein (CRP) | < 1.0 mg/dL (varies by assay) |
| Procalcitonin | < 0.1 ng/mL |
| Lactate, serum | 0.5 to 2.0 mmol/L |
Elevation patterns matter more than exact values, but recognizing what is clearly above normal will help you interpret stems quickly.
Pediatric and Pregnancy Considerations
Normal values in children and pregnant patients can differ from non pregnant adults. On USMLE questions, important exceptions are usually mentioned in the vignette. At a beginner level, you are not expected to memorize full separate pediatric and pregnancy ranges, but you should recognize that:
Pregnancy often leads to lower serum creatinine, mild anemia by dilution, and elevated alkaline phosphatase from placental production.
Infants and children can have higher heart rates, respiratory rates, and different normal hematologic values compared with adults. These are discussed in pediatric chapters.
Using Normal Values on the USMLE
Normal laboratory values on the exam are tools to identify what is abnormal in a clinical vignette and to classify that abnormality. For example, looking at sodium, potassium, and bicarbonate helps you categorize acid base disorders, while knowing that a platelet count of 40,000/µL is far below the normal 150,000 to 400,000/µL helps you recognize a bleeding risk.
Questions rarely require recalling a number to the exact decimal. Instead, you must know the approximate normal range and whether a specific value is low, normal, or high. Over time, repeated exposure to questions will help you memorize the most frequently used values, especially electrolytes, complete blood count, arterial blood gas parameters, and basic endocrine numbers.
For USMLE preparation, prioritize: CBC indices, electrolytes and anion gap, renal function, liver tests, ABGs, coagulation tests, lipid targets, and basic endocrine values. These appear across multiple disciplines and question types.
In later chapters, you will learn how to combine these normal laboratory values with patient history and physical findings to reach accurate diagnoses and choose appropriate treatments.