Measurement of PS APC-cofactor activity in mouse plasma coagulation and mouse thrombin generation assays. (A) PS APC-cofactor activity in a mouse plasma coagulation assay. Prolongation of clotting times was measured for WT (■) or Pros^+/− (▲) plasmas, to which 0.9 nM wt-rmAPC (final concentration) was added. An APC ratio was calculated as follows: (clotting times with wt-rmAPC)/(clotting times without wt-rmAPC). Mean APC ratios were 1.77 ± 0.08 for WT plasmas (n = 9) and 1.39 ± 0.01 for Pros^+/− plasmas (n = 7; P = .001). For 3 WT (□) and 2 Pros^+/− (△) plasmas, clotting times were recorded in the presence of saturating amounts of anti-PROS antibodies (0.13 mg/mL, final concentration). Mean APC ratios were 1.12 ± 0.01 and 1.09 ± 0.02 for WT and Pros^+/− plasmas, respectively. (B) The APC-cofactor activity of PS from WT or Pros^+/− plasmas was investigated in a CAT assay adapted for mouse plasma (see “Methods”). Mean thrombin generation curves were measured for WT (n = 5) and Pros^+/− (n = 6) plasmas and for each genotype, nAPCsr were calculated, as described in “Methods.” The nAPCsr were 0.88 ± 0.06 for WT (n = 5) and 2.18 ± 0.19 for Pros^+/− (n = 6) plasmas (P < .001). (C) For each genotype, thrombin generation curves were from pooled data, and mean thrombin generation curves represent the following conditions: the absence (○) and presence (△) of wt-rmAPC (3 nM, final concentration); the absence of wt-rmAPC, but in the presence of anti–human PS antibodies (0.35 mg/mL, final concentration; ▲); and the presence of 3 nM wt-rmAPC and anti–human PS antibodies (0.35 mg/mL, final concentration; ●).