A patient has serum electrolytes: Na+ 140, Cl- 95, HCO3- 15 mEq/L, with pH 7.35 and PaCO2 28 mmHg. The anion gap is 30 mEq/L. The most likely additional disturbance superimposed on the high-anion-gap metabolic acidosis is:
- A Concurrent respiratory acidosis, as PaCO2 is inappropriately high for metabolic acidosis
- B Concurrent metabolic alkalosis, as the pH is higher than expected for HAGMA alone ✓
- C No additional disturbance; this represents pure HAGMA with appropriate respiratory compensation
- D Concurrent normal anion gap metabolic acidosis due to the elevated chloride
Correct answer: B. Concurrent metabolic alkalosis, as the pH is higher than expected for HAGMA alone
Explanation
The delta-delta ratio (ΔAG/ΔHCO3-) identifies mixed disorders in high-anion-gap metabolic acidosis. ΔAG = 30 - 12 = 18; ΔHCO3- = 24 - 15 = 9. Delta ratio = 18/9 = 2.0. A delta ratio >2 suggests a concurrent metabolic alkalosis (HCO3- is higher than expected from the anion gap rise alone — i.e., the HCO3- has been elevated by a concurrent alkalotic process). Additionally, the pH of 7.35 is surprisingly near-normal despite significant metabolic acidosis, which also points to a superimposed alkalotic disorder partly correcting the pH.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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