Thrombocytopenia associated with PF3 antiplatelet activity against the sulfa component of trimethoprim-sulfamethoxazole.

A 3-year-old white boy had symptomatic thrombocytopenia after receiving trimethoprim-sulfamethoxazole in recommended doses for one week. The platelet count returned to normal soon after the medication was discontinued. In vitro admixing of donor platelets with a sample of the patient's serum demonstrated antiplatelet activity with the sulfamethoxazole component, but not with the trimethoprim component of the drug.