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.