A Rare Case of Bartter Syndrome Type 3 Diagnosed in Elderly Age

Bartter syndrome (BS) type 3 typically presents in childhood and is caused by defects in NaCl transporters of the thick ascending limb of the loop of Henle. We report a 66‐year‐old woman with asymptomatic but severe hypokalemia (2.0 mmol/L), metabolic alkalosis, and hyperreninemic hyperaldosteronism. Initial clinic blood pressure was 157/88 mmHg, but repeated office and home measurements were ~120/70 mmHg, consistent with white‐coat hypertension. Imaging excluded renovascular disease. Sequential diuretic testing supported BS physiology: a thiazide loading test increased fractional excretion of chloride (FECl) from 0.46% to 3.88% (ΔFECl 3.4%), exceeding the 2.3% cutoff that argues against Gitelman syndrome; a furosemide test showed chloride reabsorption of 10.1%, indicating a profound loop‐segment defect, even lower than values commonly reported in BS3. Targeted next‐generation sequencing identified a homozygous CLCNKB stop‐gain variant (c.1830G >A; p.Trp610Ter), confirming BS type 3. High‐dose potassium chloride (9.9 g/day) failed to correct hypokalemia, whereas low‐dose enalapril plus spironolactone normalized serum potassium (4.0 mmol/L) without persistent hypotension. The urine calcium/creatinine ratio was borderline‐normal (0.65 mmol/mmol), rather than hypocalciuric. This case suggests that BS3 should remain in the differential diagnosis of refractory hypokalemia in older adults and highlights the diagnostic value of physiology‐guided diuretic testing combined with genetic analysis, as well as the potential efficacy of low‐dose ACE inhibitor plus mineralocorticoid receptor antagonist therapy when potassium supplementation alone is insufficient.