A 50-year-old man with CKD stage 4 (eGFR 22 mL/min/1.73m²) due to diabetic nephropathy has potassium 5.8 mEq/L on repeated testing. ABG shows pH 7.28, HCO3 17 mEq/L, pCO2 38 mmHg. Urinary pH is 5.5 with a urine anion gap of +12. Which type of renal tubular acidosis (RTA) is consistent?
- A Type 1 (distal) RTA — inability to acidify urine
- B Type 4 RTA (hyperkalemic distal) — due to hypoaldosteronism or aldosterone resistance ✓
- C Type 2 (proximal) RTA — bicarbonate wasting
- D High anion gap metabolic acidosis from uremic acid accumulation
Explanation
Type 4 RTA is the most common form of RTA in clinical practice, typically caused by hypoaldosteronism (as in diabetic nephropathy — hyporeninemic hypoaldosteronism, type IV RTA) or aldosterone resistance. It is characterized by hyperkalemia, hyperchloremic normal anion gap metabolic acidosis, and low urinary ammonium excretion (positive urine anion gap). Unlike type 1 RTA, the urine pH can be appropriately acidic (<5.5) since distal H+ secretion is not completely impaired — ammoniagenesis is the defect. The low urine pH and positive urine anion gap (reflecting low NH4+ excretion) confirm type 4.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.