Drug-Induced Immunologic Thrombocytopenic Purpura

The mechanism of drug-dependent immunologic thrombocytopenic purpura (DITP) was investigated by studying the sera of four patients with classic DITP (two with quinidine-, one with acetaminophen-. and one with phenazopyridinedependent antiplatelet antibody) using a solid-phase radioimmunoassay with 125l-staphylococcal protein A. Two forms of antiplatelet antibody could be demonstrated: one that required drug to bind to platelets and one that bound to platelets in the absence of drug. Drug-dependent antiplatelet antibody required the simultaneous addition of drug and the Fc domain of the drug-dependent lgG molecule for binding to platelets. It did not require serum complement or factor VIll-related antigen for binding to platelets. Drug-dependent binding of antibody to platelets was saturation-dependent. Non-drug-dependent