Suspected trimethoprim/sulfamethoxazole-induced hypoprothrombinemia.

The following report illustrates a case of trimethoprim/sulfamethoxazole-induced hypoprothrombinemia in a patient receiving ongoing warfarin therapy for atrial fibrillation and aortic valve replacement. He was treated with trimethoprim/sulfamethoxazole (TMP/SMX) for sinusitis. During this time, the patient's prothrombin time international normalized ratio (INR) increased 3.5 times higher than the baseline value. The INR values decreased when the antibiotic was discontinued. If a patient is on warfarin and TMP/SMX is added, INR values should be monitored closely.