A 54-year-old female presents with severe metabolic acidosis. Laboratory values reveal Na+ 140 mEq/L, K+ 4.0 mEq/L, Cl- 104 mEq/L, HCO3- 12 mEq/L, arterial pH 7.22, PaCO2 28 mmHg. What is the calculated serum anion gap and the primary metabolic disturbance?

Answer and explanation

Correct answer: A. Anion gap 24 mEq/L; High anion gap acidosis with appropriate respiratory compensation

  1. Step 1: Serum Anion Gap = Na+ - (Cl- + HCO3-) = 140 - (104 + 12) = 140 - 116 = 24 mEq/L (elevated above reference 8-12 mEq/L).
  2. Step 2: Expected PaCO2 by Winter formula: 1.5 x [HCO3-] + 8 +/- 2 = 1.5(12) + 8 = 18 + 8 = 26 +/- 2 (24 to 28 mmHg). Measured PaCO2 of 28 mmHg indicates appropriate respiratory compensation.
Answer
Anion gap 24 mEq/L; High anion gap acidosis with appropriate respiratory compensation.
Why not the others: 16 mEq/L miscalculates bicarbonate subtraction; 32 mEq/L erroneously adds potassium; 12 mEq/L represents normal reference range.

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