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.