NCLEX Lab Values Guide: Normal Ranges, Critical Values, and Clinical Implications
Lab values are tested throughout the NCLEX — in pharmacology questions, fluid and electrolyte scenarios, acid-base balance, and management of care. You don't need to memorize every laboratory reference range, but you absolutely must know the high-yield values that appear repeatedly on the exam. This guide gives you the complete reference table, explains critical values, and connects each lab to its clinical meaning.
Complete NCLEX Lab Values Reference Table
| Lab Value | Normal Range | Critical Low | Critical High |
|---|---|---|---|
| Sodium (Na⁺) | 136–145 mEq/L | <120 mEq/L | >160 mEq/L |
| Potassium (K⁺) | 3.5–5.0 mEq/L | <2.5 mEq/L | >6.5 mEq/L |
| Calcium (Ca²⁺) | 8.5–10.5 mg/dL | <7.0 mg/dL | >13.0 mg/dL |
| Magnesium (Mg²⁺) | 1.5–2.5 mEq/L | <1.0 mEq/L | >9.0 mEq/L |
| Chloride (Cl⁻) | 98–106 mEq/L | <80 mEq/L | >115 mEq/L |
| BUN (Blood Urea Nitrogen) | 10–20 mg/dL | — | >100 mg/dL |
| Creatinine | 0.6–1.2 mg/dL | — | >10 mg/dL |
| Glucose (fasting) | 70–100 mg/dL | <40 mg/dL | >500 mg/dL |
| Hemoglobin (Hgb) — Female | 12–16 g/dL | <7 g/dL | >20 g/dL |
| Hemoglobin (Hgb) — Male | 14–18 g/dL | <7 g/dL | >20 g/dL |
| Hematocrit (Hct) — Female | 37–47% | <21% | >65% |
| Hematocrit (Hct) — Male | 42–52% | <21% | >65% |
| Platelets | 150,000–400,000/mm³ | <50,000/mm³ | >1,000,000/mm³ |
| WBC (White Blood Cells) | 5,000–10,000/mm³ | <2,000/mm³ | >30,000/mm³ |
| PT (Prothrombin Time) | 11–13 seconds | — | >20 sec (critical) |
| INR (normal, not on anticoag) | 0.8–1.2 | — | >4.0 (critical) |
| ABG: pH | 7.35–7.45 | <7.20 | >7.60 |
| ABG: PaCO₂ | 35–45 mmHg | <20 mmHg | >70 mmHg |
| ABG: PaO₂ | 80–100 mmHg | <40 mmHg | — |
| ABG: HCO₃⁻ | 22–26 mEq/L | <10 mEq/L | >40 mEq/L |
Understanding Critical Values
A critical value is a lab result so far outside the normal range that it represents immediate risk to the patient's life. When a critical value is reported, the nurse must notify the physician or provider immediately and document the communication.
Potassium: The NCLEX Favorite
Potassium is tested more than any other electrolyte. Both hypokalemia and hyperkalemia cause life-threatening cardiac dysrhythmias:
- Hypokalemia (<3.5 mEq/L): Muscle weakness, cramps, constipation, U waves on EKG, increased digoxin toxicity risk. Causes: diuretics, vomiting, diarrhea.
- Hyperkalemia (>5.0 mEq/L): Peaked T waves, widened QRS, bradycardia, cardiac arrest. Causes: renal failure, ACE inhibitors, potassium-sparing diuretics, cell lysis.
Sodium: Neuro Impact
- Hyponatremia (<136 mEq/L): Headache, confusion, seizures, coma. Severe (<120) = seizure risk. Correct slowly to avoid osmotic demyelination.
- Hypernatremia (>145 mEq/L): Thirst, restlessness, dry mucous membranes, neurological changes. Caused by fluid loss or excess sodium intake.
Calcium: Muscle and Nerve Function
- Hypocalcemia (<8.5 mg/dL): Trousseau's sign (carpopedal spasm with BP cuff), Chvostek's sign (facial twitching with nerve tap), tetany, seizures. Post-thyroidectomy risk.
- Hypercalcemia (>10.5 mg/dL): "Bones, groans, stones, psychic moans" — bone pain, constipation, renal stones, confusion. Associated with hyperparathyroidism and malignancy.
ABG Interpretation: ROME Method
Use the ROME mnemonic to interpret ABGs: Respiratory Opposite, Metabolic Equal
- Respiratory acidosis: pH ↓, PaCO₂ ↑ (opposite direction). Cause: hypoventilation (COPD, sedation).
- Respiratory alkalosis: pH ↑, PaCO₂ ↓ (opposite). Cause: hyperventilation (anxiety, PE, mechanical ventilation).
- Metabolic acidosis: pH ↓, HCO₃⁻ ↓ (same direction). Cause: DKA, renal failure, lactic acidosis.
- Metabolic alkalosis: pH ↑, HCO₃⁻ ↑ (same direction). Cause: vomiting, NG suctioning, antacid overuse.
NCLEX Practice Question
Question: The nurse reviews morning lab results for a client receiving furosemide. Which result requires immediate notification of the physician?
A) Sodium 138 mEq/L
B) Potassium 2.4 mEq/L
C) BUN 18 mg/dL
D) Creatinine 0.9 mg/dL
Correct Answer: B
Rationale: A potassium of 2.4 mEq/L is below the critical threshold (<2.5 mEq/L) and represents a critical value requiring immediate physician notification. Furosemide is a potassium-wasting diuretic, making hypokalemia a known and dangerous side effect. Hypokalemia can cause life-threatening cardiac dysrhythmias. All other values (A, C, D) are within normal range.
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