A 55-year-old woman with type 1 RTA has a non-anion gap metabolic acidosis and hyperchloraemia. Urine anion gap is positive (+12 mEq/L). Urine pH is 6.5 despite systemic acidosis (pH 7.28). The urine anion gap indicates:

• A Impaired urinary NH4⁺ excretion, consistent with distal RTA ✓
• B Adequate urinary NH4⁺ excretion, suggesting proximal RTA or GI bicarbonate loss
• C Anion gap metabolic acidosis
• D Normal renal acidification with extrarenal cause

Explanation

Urine anion gap (UAG) = Na⁺ + K⁺ − Cl⁻; a positive UAG indicates low urinary ammonium (NH4⁺) because NH4⁺ is the main unmeasured cation that drives a negative UAG when renal NH4⁺ excretion is intact. A positive UAG in the context of non-anion gap metabolic acidosis indicates impaired urinary NH4⁺ secretion, pointing to distal (type 1) RTA. A negative UAG indicates high NH4⁺ and normal renal acidification, as occurs in diarrhoea-induced bicarbonate loss or proximal RTA (where distal acidification remains intact). A urine pH of 6.5 in the setting of systemic acidosis confirms distal acidification failure.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.