Sulfisoxazole-induced thrombocytopenic purpura. Immunologic mechanism as cause.

During treatment of brucellosis with sulfisoxazole, tetracycline, and streptomycin sulfate, severe thrombocytopenic purpura developed in a young farmer. Verification for an immune mechanism was provided by clinical challenge with a small dose of sulfisoxazole that caused recurrence of thrombocytopenia and by serologic laboratory test results that detected a serum factor causing platelet agglutination requiring the presence of sulfisoxazole. The original antigenic stimulation was considered to come from drinking cows' milk contaminated with sulfonamide drugs. Cross-reactivity with some other sulfonamide drugs was demonstrated.