A patient has pH 7.30, PaCO2 28 mmHg, HCO3− 13 mEq/L, Na+ 140, Cl− 112. The calculated anion gap (AG) is: (pick calculation from options). Using a normal AG of 12, is there a concurrent metabolic alkalosis?
- A AG = 15; corrected delta-delta ratio = (15−12)/(24−13) = 0.27, indicating concurrent metabolic alkalosis is NOT present
- B AG = 15; delta-delta ratio = (15−12)/(24−13) = 0.27, indicating a pure high-AG metabolic acidosis with maximal respiratory compensation
- C AG = 18; delta-delta ratio = (18−12)/(24−13) = 0.55, indicating a mixed disorder
- D AG = 15; delta-delta ratio 0.27, indicating a concurrent non-AG metabolic acidosis ✓
Explanation
AG = Na − (Cl + HCO3) = 140 − (112 + 13) = 15 mEq/L. Excess AG = 15 − 12 = 3. Expected HCO3 if only high-AG acidosis = 24 − 3 = 21 mEq/L. Measured HCO3 is only 13, which is lower than the expected 21. This means additional HCO3 has been consumed beyond what the AG gap alone accounts for, indicating a concurrent non-AG (normal AG/hyperchloraemic) metabolic acidosis. The delta-delta (delta ratio) = change in AG / change in HCO3 = 3/11 ≈ 0.27; a ratio <1 indicates coexistent non-AG metabolic acidosis. Respiratory compensation for metabolic acidosis: expected PaCO2 = 1.5 × 13 + 8 = 27.5 ± 2; actual PaCO2 = 28, so respiratory compensation is appropriate.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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