We read with great interest the recently reported [1] association between high-dose trimethoprim-sulfamethoxazole therapy and hyperkalemia seen in patients with AIDS. Although our patient population differs substantially from that studied by Greenberg and colleagues [1], we have seen a similar clinical course evolve in an elderly, cognitively impaired patient who was being treated for a urinary tract infection. Our patient, a 76-year-old woman, presented with normal serum electrolyte levels (sodium, 136 mmol/L; potassium, 4.5 mmol/L; HCO₃, 24 mmol/L; chlorine, 108 mmol/L; blood urea nitrogen, 23 mg/dL; and creatinine, 1.1 mg/dL) and no known history of renal, endocrine, or metabolic abnormalities. At admission, urinalysis was similarly unremarkable with the exception of pyuria and bacteruria. Subsequent culture yielded a pan-sensitive Klebsiella species, and the patient was given trimethoprim-sulfamethoxazole (Septra-DS; Burroughs Wellcome, Research Triangle Park, North Carolina), one tablet twice daily, beginning on hospital day 3. One week later, her mental status declined acutely, and her electrolytes showed a serum potassium level of 6.2 mmol/L, without evidence of azotemia, acidosis, or dehydration. The patient had not received any other known nephrotoxic medications during this period. Her urinary sediment continued to be unremarkable. Therapy with trimethoprim-sulfamethoxazole was discontinued, and the patient was treated with one dose of kayexalate. Her serum potassium level returned to normal and remained so for the duration of her 3-week hospitalization.

To our knowledge, no available data exist on the incidence of hyperkalemia in geriatric patients receiving standard adult doses of this commonly used antibiotic. Although the mechanism by which trimethoprim-sulfamethoxazole induces hyperkalemia remains unclear, does the age-associated decrease in the glomerular filtration rate place our elderly patients at higher risk for a dose-dependent, potentially fatal complication? Until additional data are available, we suggest using this antibiotic treatment with greater caution in elderly patients.