Ampicillin-dependent IgG Antibody Mediated Thrombocytopenia Confirmed Using Flow Cytometry.

Drug-induced immune thrombocytopenia (DITP) caused by ampicillin (ABPC) is rarely reported. An 87-year-old woman developed severe thrombocytopenia on postoperative day 3 during treatment with multiple drugs, including ABPC, for chronic periprosthetic infection following total knee arthroplasty. Although the platelet counts were unresponsive to oral prednisolone and intravenous immunoglobulin, they recovered promptly after the discontinuation of ABPC. Flow cytometry demonstrated ABPC-dependent IgG binding to healthy donor platelets. This is the first report to confirm ABPC-dependent platelet-reactive IgG using flow cytometry, thereby expanding the diagnostic tools for suspected DITP and highlighting the need for early recognition, prompt drug withdrawal, and appropriate supportive care.