Lab values you must know cold
The reference values that appear constantly across Reduction of Risk Potential and Pharmacology questions. Ranges below are the commonly taught NCLEX reference ranges — always follow the range given in the question itself.
| Lab | Normal range | Why it's tested |
|---|---|---|
| Sodium (Na⁺) | 135–145 mEq/L | Neuro changes at both extremes; correct slowly |
| Potassium (K⁺) | 3.5–5.0 mEq/L | Cardiac rhythm; peaked T waves high, U waves low |
| Calcium (Ca⁺⁺) | 9.0–10.5 mg/dL | Tetany when low (Chvostek / Trousseau signs) |
| Magnesium (Mg⁺⁺) | 1.3–2.1 mEq/L | Deep tendon reflexes; watch in preeclampsia therapy |
| Chloride (Cl⁻) | 98–106 mEq/L | Moves with sodium; acid-base balance |
| Glucose (fasting) | 70–100 mg/dL | Hypoglycemia is the acute emergency |
| Creatinine | 0.6–1.2 mg/dL | Kidney function; check before nephrotoxic drugs & contrast |
| BUN | 10–20 mg/dL | Rises with dehydration and renal impairment |
| Hemoglobin | 12–16 (F) / 14–18 (M) g/dL | Anemia and bleeding assessment |
| Hematocrit | 37–47 (F) / 42–52 (M) % | Roughly 3× the hemoglobin value |
| Platelets | 150,000–400,000/mm³ | Bleeding precautions below ~50,000 |
| WBC | 5,000–10,000/mm³ | Infection when high; neutropenic precautions when low |
| pH (arterial) | 7.35–7.45 | The anchor of every ABG question |
| PaCO₂ | 35–45 mm Hg | Respiratory component of acid-base |
| HCO₃ | 22–26 mEq/L | Metabolic component of acid-base |
| INR (on warfarin) | 2.0–3.0 | Therapeutic anticoagulation target |
| aPTT (on heparin) | 1.5–2.5× control | Monitors heparin, not warfarin |
| Digoxin level | 0.5–2.0 ng/mL | Toxicity risk rises with hypokalemia |
| Lithium level | 0.6–1.2 mEq/L | Narrow margin; toxicity with dehydration |
| HbA1c | < 5.7% (normal) | 3-month glucose average; diabetes control |
