A 60-year-old diabetic is admitted with an arterial blood gas showing: pH 7.22, PaCO2 20 mmHg, HCO3- 8 mEq/L, Na+ 138, Cl- 100, glucose 180 mg/dL. Serum lactate is normal; urine ketones are negative. What is the MOST likely acid-base disorder and its cause?
- A High anion gap metabolic acidosis due to lactic acidosis
- B Normal anion gap (hyperchloraemic) metabolic acidosis due to renal tubular acidosis
- C High anion gap metabolic acidosis due to uraemic organic acid accumulation in chronic kidney disease ✓
- D Mixed metabolic and respiratory acidosis
Correct answer: C. High anion gap metabolic acidosis due to uraemic organic acid accumulation in chronic kidney disease
Explanation
Anion gap = Na+ - (Cl- + HCO3-) = 138 - (100 + 8) = 30 mEq/L (high, normal 8-12). Lactate normal, ketones negative — this is uraemic acidosis. In CKD, accumulation of unmeasured organic anions (sulphate, phosphate, hippurate, indoxyl sulphate) from impaired renal excretion elevates the anion gap. PaCO2 of 20 represents appropriate respiratory compensation (Winter's formula: expected PaCO2 = 1.5 × HCO3- + 8 ± 2 = 20 ± 2), confirming a simple high anion gap metabolic acidosis.
Reference: Harper's Illustrated Biochemistry, 32nd ed.
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