The effects of aPL IgG on the quantity of platelet surface annexin-V and plasma coagulation. (A) Washed human platelets were exposed to three different aPL and control IgG preparations in plasma, following which the platelets were incubated with annexin-V (20 μg/mL), in the presence of calcium, as described in Materials and Methods. Surface annexin-V was then dissociated with EGTA and measured by ELISA. Platelets preexposed to aPL IgG had significantly less annexin-V on their surfaces (mean ± SEM, 0.89 ± 0.12 ng/10⁶ platelets) as compared with controls (2.01 ± 0.38 ng/10⁶ platelets, P = .05). (B) Plasma coagulation times were determined using platelets, which had been preexposed to the aPL and control IgGs in plasma. The platelets were added to pooled normal plasma, which was recalcified in the presence and absence of added annexin-V (20 μg/mL). Annexin-V lengthened the coagulation times of pooled normal plasma with both control and aPL IgG-treated platelets. The net prolongation, compared with the coagulation time without annexin-V, was significantly less with the aPL-treated platelets (mean prolongation ± SEM, 33.2 ± 0.9 seconds) as compared with controls (50.4 ± 4.1 seconds, n = 3, P = .01).