A 25-year-old woman presents with weakness and hypokalaemia (K+ 2.8 mEq/L). Urine pH is consistently 6.2. Arterial blood gas shows pH 7.28, HCO3 13 mEq/L, PaCO2 30 mmHg. Urine potassium-to-creatinine ratio is elevated. Which renal tubular acidosis does this represent and what is its most common cause in young women?

• A Type 2 (proximal) RTA; most common cause is multiple myeloma
• B Type 4 RTA; caused by aldosterone deficiency
• C Type 2 (proximal) RTA; characterised by urine pH that can fall below 5.5 with acidosis
• D Type 1 (distal) RTA; most common cause is Sjogren's syndrome or autoimmune conditions ✓

Explanation

Type 1 (distal) RTA is characterised by inability to acidify urine below pH 5.5 (here pH 6.2 despite acidaemia), hypokalaemia, normal anion gap metabolic acidosis, and urinary potassium wasting. In young women, primary Sjogren's syndrome is the most common secondary cause of distal RTA due to interstitial nephritis impairing H+-ATPase in collecting duct alpha-intercalated cells. Type 4 RTA causes hyperkalaemia; proximal RTA urine pH can fall below 5.5 when bicarbonate wasting is complete.

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

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Written and medically reviewed by the StethoPrep medical team.