Which formula correctly calculates the serum anion gap from standard serum electrolyte measurements?
Answer and explanation
Correct answer: B. Sodium minus the sum of chloride and bicarbonate
- Step 1: The serum anion gap represents unmeasured anions in plasma (such as albumin, phosphate, and organic acids).
- Step 2: It is calculated by subtracting the sum of measured anions from measured sodium: Anion Gap = Na - (Cl + HCO3), with a normal reference range of 8 to 12 mEq/L.
Answer
Sodium minus the sum of chloride and bicarbonate.
Why not the others: Sodium plus potassium minus chloride ignores bicarbonate and does not calculate the anion gap. Chloride minus bicarbonate divided by sodium is an arbitrary ratio without physiological relevance. Multiplying sodium by chloride has no clinical meaning in acid-base evaluation.
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