The effects of aPL plasmas on annexin-V bound to prothrombin time reagent (tissue factor-phospholipid complex) and on plasma coagulation. (A) Prothrombin time reagent was preexposed to three different aPL and control IgG preparations in plasma and then to annexin-V (2 μg/mL), after which surface annexin-V was dissociated with EDTA and measured by ELISA. Prothrombin time reagent preexposed to aPL IgG-containing plasmas had significantly less annexin-V (mean ± SEM, 82 ± 4 ng/50 μL aliquot of reagent) compared with controls (110 ± 1 ng/50 μL aliquot of reagent, P = .02). (B) Plasma coagulation times were determined using prothrombin time reagent, which was exposed to the aPL and control plasmas (n = 10 for each group), in the first stage and in the second stage to pooled normal plasma in the presence and absence of annexin-V (30 μg/mL). There was a small but significant prolongation of coagulation time when the prothrombin time reagent was exposed to the aPL plasmas in the absence of annexin-V (P = .003). Addition of annexin-V resulted in prolongation of coagulation times with both types of reagent (ie, exposure to control and aPL plasmas). However, in contrast to the results without annexin-V, the coagulation times of the prothrombin time reagent, which had been preexposed to aPL-plasma, were significantly shortened (mean ± SEM, 35.0 ± 0.8 seconds compared with 38.3 ± 1.2 seconds for control plasmas, P = .03). There was a concomitant significant decrease in the net prolongation of the coagulation times using PT reagent, which had been pretreated with aPL plasma (10.3 ± 0.8 seconds compared with 15.2±1.2 seconds for control plasmas, P = .004).