Effect of FV level on the APCsr determined with the Coatest APC Resistance assay. (A) Plot of the APCsr as a function of FV concentration. The FV level was varied by mixing sample plasma with FV-deficient plasma in different proportions, and the APCsr was determined as described in “Materials and methods.” (B) The difference in APCsr between normal plasma and FV_Leiden-homozygous plasma consists of 2 components: (1) poor susceptibility of FVa_Leiden to APC, quantified by the difference between the y-axis intercepts of the APCsr plots of normal and FV_Leiden-homozygous plasma; and (2) impaired APC cofactor activity of FV_Leiden in FVIIIa inactivation, whose contribution to the APCsr is deduced by subtracting the susceptibility component from the total APCsr difference between normal and FV_Leiden-homozygous plasma measured in full plasma. (C) Magnification of the inset in panel A. Each point is the average of 3 or more duplicate experiments, with error bars representing 1 standard error above and below the mean. Δ indicates normal plasma; ▪, FV_R2-homozygous plasma; and •, FV_Leiden-homozygous plasma.