A 28-year-old woman with type 1 diabetes presents obtunded. ABG: pH 7.22, PaCO2 18 mmHg, HCO3- 7 mEq/L. Na+ 138, Cl- 98 mEq/L. Calculate the anion gap and delta ratio.
- A Anion gap = 20 mEq/L; delta ratio = 0.5 — mixed high-AG and normal-AG acidosis
- B Anion gap = 33 mEq/L; delta ratio = 2.5 — high-AG acidosis with coexistent metabolic alkalosis
- C Anion gap = 33 mEq/L; delta ratio = (33-12)/(24-7) = 21/17 = 1.24 — pure high-AG metabolic acidosis (DKA) ✓
- D Anion gap = 43 mEq/L; delta ratio = 1.0 — pure high-AG metabolic acidosis
Correct answer: C. Anion gap = 33 mEq/L; delta ratio = (33-12)/(24-7) = 21/17 = 1.24 — pure high-AG metabolic acidosis (DKA)
Explanation
Anion gap = Na+ − (Cl− + HCO3−) = 138 − (98 + 7) = 33 mEq/L (normal 12 ± 2). Delta ratio = (AG − 12) / (24 − HCO3−) = (33 − 12) / (24 − 7) = 21 / 17 = 1.24. A delta ratio between 1 and 2 indicates pure high-AG metabolic acidosis without coexistent normal-AG acidosis (ratio < 1) or metabolic alkalosis (ratio > 2). DKA with ketoanions explains both the elevated AG and the delta ratio in this range. Expected respiratory compensation: PaCO2 = 1.5 × 7 + 8 ± 2 = 18.5 mmHg (actual 18 mmHg — appropriate).
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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