NCLEX Lab Values Cheat Sheet: Critical Ranges Every Nurse Must Know

Lab values are woven into nearly every NCLEX question. You might not get a question that directly asks "What is the normal potassium level?" — instead, the lab value will be embedded in a clinical scenario and you will need to recognize whether it is normal, high, or low and then choose the correct nursing action. Knowing these ranges cold is essential.

Focus on what the value means, not just the number

The NCLEX tests your ability to interpret lab results in context. A potassium of 5.8 mEq/L means you need to check for cardiac dysrhythmias and hold potassium-sparing drugs — that clinical connection is what the exam is really testing.

Electrolytes

Sodium (Na+): 136–145 mEq/L. Hyponatremia causes confusion, seizures, and muscle cramps. Hypernatremia causes thirst, dry mucous membranes, and agitation. Potassium (K+): 3.5–5.0 mEq/L. Hypokalemia causes muscle weakness, leg cramps, and dangerous dysrhythmias. Hyperkalemia causes peaked T waves, bradycardia, and cardiac arrest. Always place the patient on a cardiac monitor for critical potassium abnormalities.

Calcium (Ca2+): 9.0–10.5 mg/dL. Hypocalcemia causes Trousseau's sign (carpopedal spasm) and Chvostek's sign (facial twitching). Hypercalcemia causes lethargy, constipation, and kidney stones. Magnesium (Mg2+): 1.3–2.1 mEq/L. Hypomagnesemia mirrors hypocalcemia and often occurs alongside it.

Renal Function

BUN: 10–20 mg/dL. Creatinine: 0.7–1.3 mg/dL. Elevated BUN and creatinine together suggest kidney dysfunction. A rising creatinine is especially significant. GFR below 60 mL/min for three or more months indicates chronic kidney disease.

Hematology

Hemoglobin: 12–18 g/dL (varies by sex). Hematocrit: 37–52%. WBC: 5,000–10,000/mm³. Platelets: 150,000–400,000/mm³. A platelet count below 50,000 increases bleeding risk — implement bleeding precautions. A WBC above 10,000 suggests infection; below 4,000 suggests immunosuppression — implement neutropenic precautions.

Know the critical values that require immediate action

Potassium below 3.0 or above 6.0, sodium below 120 or above 160, glucose below 50 or above 400, and platelets below 50,000 — these are values where you notify the provider immediately and anticipate urgent interventions.

Coagulation Studies

PT: 11–13.5 seconds. INR: 0.8–1.1 (therapeutic on warfarin: 2.0–3.0). aPTT: 30–40 seconds (therapeutic on heparin: 1.5–2.5x normal). If INR is above 4.5 or aPTT is above 100 seconds, hold the anticoagulant and notify the provider.

Metabolic Panel

Glucose (fasting): 70–100 mg/dL. A1C: below 7% for diabetic management. ABG normals: pH 7.35–7.45, PaCO2 35–45 mmHg, HCO3 22–26 mEq/L, PaO2 80–100 mmHg. Learn the ROME mnemonic for ABG interpretation: Respiratory Opposite, Metabolic Equal.

Write the critical lab values on a whiteboard you see every day. Within a week you will have them memorized. Repetition through daily exposure beats cramming every time.

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