A patient with chronic diarrhoea has: pH 7.32, PaCO2 28 mmHg, HCO3 14 mEq/L, Na 140, Cl 112, K 3.1. Calculate the anion gap and classify the acid-base disorder:

• A Anion gap 14; high anion gap metabolic acidosis
• B Anion gap 18; high anion gap metabolic acidosis with respiratory alkalosis
• C Anion gap 14; normal anion gap metabolic acidosis with appropriate respiratory compensation ✓
• D Anion gap 12; normal anion gap metabolic acidosis — cannot classify compensation

Explanation

Anion gap = Na − (Cl + HCO3) = 140 − (112 + 14) = 140 − 126 = 14 mEq/L (normal range 8–12, some labs 12–16; this is borderline normal/mildly elevated). Diarrhoea causes direct HCO3 loss (hyperchloraemic/normal anion gap metabolic acidosis). Winter's formula: expected PaCO2 = (1.5 × 14) + 8 ± 2 = 21 + 8 = 29 ± 2, range 27–31 mmHg. Measured PaCO2 of 28 is within range → appropriate respiratory compensation. Classification: normal (hyperchloraemic) anion gap metabolic acidosis with appropriate compensation.

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

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