A patient's arterial blood gas shows: pH 7.30, PaCO₂ 24 mmHg, HCO₃⁻ 11 mEq/L. Serum electrolytes: Na⁺ 140, Cl⁻ 116, K⁺ 4 mEq/L. What is the anion gap and what is the most likely acid-base disorder?

• A AG = 24; high anion gap metabolic acidosis with appropriate respiratory compensation
• B AG = 13; normal anion gap metabolic acidosis ✓
• C AG = 13; mixed metabolic acidosis and respiratory alkalosis
• D AG = 18; high anion gap metabolic acidosis

Explanation

Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) = 140 − (116 + 11) = 13 mEq/L, which is normal (normal AG = 12 ± 2). This is a normal-anion-gap (hyperchloremic) metabolic acidosis. Expected respiratory compensation by Winter's formula: expected PaCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 = 1.5(11) + 8 = 24.5 mmHg, closely matching the measured 24 mmHg—confirming appropriate compensation without a superimposed respiratory disorder.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.