Patient plasma samples contained drug-dependent antibodies.

Plasma from patients T1 (A) and E1 (B) contained IgG antibodies that reacted with platelets only in the presence of the GPIIb/IIIa inhibitors tirofiban and eptifibatide, respectively (open histograms). No reaction was obtained in the absence of drug (closed histograms) or with normal serum in the presence or absence of drug (not shown). Serum from the other 7 patients gave reactions similar to those shown for patients T1 and E1.