Effect of Standard-Dose Trimethoprim/Sulfamethoxazole on the Serum Potassium Concentration in Elderly Men

Abstract

OBJECTIVE: To determine the effect of standard-dose trimethoprim/sulfamethoxazole (TMP/SMX) (TMP 160 mg and SMX 800 mg q12h) on the serum potassium concentration. DESIGN: Retrospective and concurrent study. SETTING: A Veterans Affairs Medical Center. PATIENTS: Fifty-three men hospitalized at the Fargo Veterans Affairs Medical Center. Thirty-three patients who received standard-dose TMP/SMX for 3 or more days comprised the study group. Twenty patients who received oral cephradine or amoxicillin for 3 or more days comprised the control group. Patients who received potassium supplements, potassium-sparing diuretics, angiotensin-converting enzyme inhibitors, nonsteroidal antiinflammatory drugs, beta-blockers, heparin, known nephrotoxic agents, patients with a serum creatinine concentration of more than 177 μmol/L, and patients with baseline hyperkalemia (serum potassium concentration>5.1 mmol/L) were excluded. RESULTS: The serum potassium concentration in the study group was 4.22 ± 0.40 mmol/L and increased by 0.31 ± 0.38 mmol/L at the end of therapy (p < 0.001). Twenty-six patients in the study group (78.8%) had an increase in the serum potassium concentration during TMP/SMX therapy. Fourteen of these patients had follow-up serum potassium concentrations obtained after completion of therapy. The serum potassium concentration returned to baseline in 10 of these patients. The serum creatinine concentration also increased during therapy. However, the correlation between the increase in the serum potassium concentration and the increase in the serum creatinine concentration was weak (Pearson r = 0.29). The serum potassium in the control group was 4.34 mmol/L and remained essentially unchanged during therapy. CONCLUSIONS: Therapy with standard-dose TMP/SMX is associated with a slight increase in the serum potassium concentration. Routine monitoring of the serum potassium concentration in patients who are treated with standard-dose TMP/SMX therapy is unnecessary. However, TMP/SMX should be considered as a possible cause of unexplained hyperkalemia in elderly patients receiving TMP/SMX therapy.