A patient presents with renal tubular acidosis. Lab results: serum Na+ 138, K+ 3.1 mEq/L, Cl− 112 mEq/L, HCO3− 14 mEq/L, pH 7.30. Urine pH is 5.5. Urine anion gap (UAG = urine Na+ + urine K+ − urine Cl−) is −15 mEq/L. Which type of RTA does this pattern indicate?
- A Type 1 (distal) RTA — urine pH fixed > 5.5, positive UAG indicating impaired NH4+ excretion
- B Type 4 (hyperkalemic) RTA — characterised by hyperkalemia and metabolic acidosis
- C Type 2 (proximal) RTA — urine pH < 5.5 when HCO3 is low, negative UAG indicating intact NH4+ excretion ✓
- D Type 3 (mixed) RTA — combined proximal and distal defect
Explanation
In proximal (Type 2) RTA, the defect is in proximal tubular HCO3 reabsorption. When plasma HCO3 falls below the reduced threshold (typically 15–18 mEq/L in proximal RTA), the distal tubule can still acidify urine normally, so urine pH falls below 5.5. The UAG reflects net urinary NH4+ excretion: a negative UAG (urine Cl− > Na+ + K+) indicates appropriate NH4+ excretion (NH4+ carries the positive charge, so Cl− must be high to maintain electroneutrality). This is consistent with intact distal acidification and proximal bicarbonate wasting. In distal (Type 1) RTA, urine pH is persistently > 5.5 even in acidaemia, and UAG is positive (impaired NH4+ excretion). Type 4 RTA is characterised by hyperkalaemia and low urine NH4+ (hyperaldosteronism-related).
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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