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Sodium (Na+)
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135–145 mEq/L — critical <120 or >160
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Potassium (K+)
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3.5–5.0 mEq/L — critical <2.5 or >6.0; check ECG (peaked T / U waves)
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Calcium (Ca2+)
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8.5–10.5 mg/dL — Trousseau's/Chvostek's signs when low
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Magnesium (Mg2+)
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1.5–2.5 mEq/L — low Mg worsens refractory hypokalemia
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Phosphorus
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2.5–4.5 mg/dL
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Chloride (Cl−)
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98–106 mEq/L
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BUN
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10–20 mg/dL — rises with dehydration and renal failure
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Creatinine
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0.6–1.2 mg/dL — best single indicator of renal function
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BUN:Creatinine ratio
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~10:1 normal; high ratio suggests prerenal cause (dehydration)
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Glucose (fasting)
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70–100 mg/dL — critical <40 or >400
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HbA1c
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<5.7% normal; ≥6.5% diabetes
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WBC
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5,000–10,000/mm³ — high = infection/inflammation
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RBC
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M 4.5–5.5 / F 4.0–5.0 million/mm³
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Hemoglobin
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M 14–18 / F 12–16 g/dL — critical <5 or >20
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Hematocrit
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M 40–54% / F 36–46% — roughly 3× hemoglobin
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Platelets
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150,000–400,000/mm³ — critical <20,000 (bleeding risk)
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PT
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11–12.5 sec — monitors warfarin with INR
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INR
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0.8–1.1 normal; therapeutic on warfarin 2.0–3.0 — critical >5
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aPTT
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30–40 sec — monitors heparin; therapeutic 1.5–2.5× normal
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pH
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7.35–7.45
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PaCO₂
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35–45 mmHg — respiratory component (up = acidosis)
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HCO₃⁻
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22–26 mEq/L — metabolic component
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PaO₂
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80–100 mmHg; SaO₂ 95–100%
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Quick read: match the abnormal value to its system — CO₂ abnormal = respiratory, HCO₃ abnormal = metabolic; pH tells acidosis vs alkalosis
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Digoxin
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0.8–2.0 ng/mL — toxicity: vision changes, anorexia, arrhythmias; check K+ first
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Lithium
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0.6–1.2 mEq/L — toxicity >1.5; maintain hydration and salt intake
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Phenytoin (Dilantin)
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10–20 mcg/mL — toxicity: nystagmus, ataxia, slurred speech
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Theophylline
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10–20 mcg/mL — toxicity: tachycardia, seizures
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K+ <2.5 or >6.0, Na+ <120 or >160, platelets <20,000, Hgb <5, glucose <40 or >400, INR >5
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NCLEX rule: unstable airway/breathing/circulation beats lab values — assess the patient, then report critical labs immediately
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