Detecting APC-resistant factor V: a functional method without plasma dilution.

A method for detecting activated protein C (APC)-resistant factor V, especially factor V Leiden, is described, which uses reagents containing two unfractionated snake venoms. The procedure can be used for testing plasma samples from patients receiving oral anticoagulant therapy, heparin therapy and patients with lupus anticoagulant, and does not require the use of factor-V-deficient plasma. The sample plasma is first incubated with dilute venom from Agkistrodon contortrix contortrix (Southern Copperhead) which activates the endogenous protein C and then a dilute Russell's viper venom time test is performed. In individuals with APC-resistant factor V, especially factor V Leiden, a marginal prolongation of dilute Russell's viper venom time was noted [1.14 +/- 0.14 s (n = 16)]. Non-carriers were easily discriminated in each patient group, with a prolongation of 2.69 +/- 0.30 s for normal blood donors (n = 127), 2.61 +/- 0.38 s for patients taking oral anticoagulants (n = 102), 2.41 +/- 0.45 s for patients taking heparin (n = 96), and 2.38 +/- 0.41 s for patients with lupus anticoagulant (n = 22). Patients taking oral anticoagulants with moderate prolongation (between 1.5- and 2.0-fold) may have low levels of functional protein C and this might additionally indicate a subgroup of such patients at higher than normal thrombotic risk.