A patient is found to have a urine pH of 6.2, serum bicarbonate of 14 mmol/L, and normal anion gap metabolic acidosis. Urinary ammonium excretion (estimated from urine anion gap) is positive (+12 mmol/L). What type of renal tubular acidosis is present?

• A Type 1 RTA (distal) — failure to acidify urine
• B Type 4 RTA (hyperkalaemic) — impaired ammoniagenesis ✓
• C Type 2 RTA (proximal) — bicarbonate wasting
• D No RTA — this is a normal variant

Explanation

A positive urinary anion gap (UAG = urine Na + K − Cl) indicates low urinary ammonium excretion (because NH4⁺ accompanies Cl⁻; a large negative UAG implies high NH4⁺). A positive UAG suggests impaired ammonium production, which is the hallmark of Type 4 RTA (hyperreninaemic hypoaldosteronism or aldosterone resistance), where impaired aldosterone action reduces NH4⁺ synthesis. The urine pH of 6.2 confirms the kidney can acidify (excluding classic type 1 distal RTA). Type 1 RTA shows urine pH persistently > 5.5. Type 2 RTA shows a negative UAG (appropriate NH4⁺ excretion) with bicarbonate wasting.

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

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