Sulfamethoxazole-induced thrombocytopenia masquerading as posttransfusion purpura: a case report.

Background: Drug-induced immune thrombocytopenia (DITP) is a rare clinical disorder characterized by accelerated platelet (PLT) clearance in the presence of drug-dependent antibodies. Distinguishing DITP from other immune-mediated disorders such as posttransfusion purpura (PTP) and autoimmune thrombocytopenia can represent a clinical challenge.

Case Report: A 68-year-old male with no prior transfusion history presented to the emergency department (ED) with dyspnea, epistaxis, and severe thrombocytopenia (<10 × 10(9)/L) 12 days after discharge from a hospital admission for a coronary artery bypass graft. Evaluation of the degree of thrombocytopenia and the temporal association between the peri- and postoperative receipt of multiple transfusions and the acute decrease in PLT count indicated PTP as a possible cause of the severe thrombocytopenia. Treatment with 1 g/kg intravenous immunoglobulin (IVIG) was initiated and followed by a rapid 48-hour increase in the PLT count. PLT antibodies lacking serologic specificity were subsequently identified in a sample collected upon presentation. Two weeks later he again presented to the ED with epistaxis and severe thrombocytopenia (<10 × 10(9)/L). Clinical history now revealed that the patient had been treated with trimethoprim-sulfamethoxazole by his primary care physician after his first hospitalization for a "cellulitic-appearing" leg and again before his final presentation for surgical site erythema and edema. IVIG was administered again with a rapid return of PLT count to baseline. Sulfamethoxazole-dependent PLT antibodies were subsequently identified in the original patient sample.

Conclusion: This case report documents a case of IVIG-responsive DITP initially misdiagnosed as PTP, highlighting the clinical overlap of these immunologic-mediated phenomena.