The Factor V (Leiden) test: evaluation of an assay based on dilute Russell Viper Venom Time for the detection of the Factor V Leiden mutation

In the present study a new clotting assay for the detection of an increased resistance of coagulation factor V against degradation by activated protein C (Factor V Leiden mutation, FVLM) was evaluated. The Factor V (Leiden) Test (Gradipore, North Ryde NSW, Australia) is based on the dilute Russell Viper Venom Time (DRVVT), which is prolonged when the plasma sample is preincubated with dilute whole Agkistrodon contortrix contortrix venom for activation of protein C (PC). In contrast to the DRVVT based global assay, Protein C Pathway Test (Gradipore, North Ryde NSW, Australia) this new assay is expected to be more specific for FVLM because of optimized amounts of the venom. The test result is expressed as the ratio between the DRVVT with and without addition of the venom. The following precision values were found: intraassay coefficient of variation (CV): 5.53% (n=20) in the normal range, 4.30% (n=20) in the pathological range; interassay CV: 6.90% (n=10) and 7.64% (n=10), respectively. A normal range (5th to 95th percentile) of 2.12 to 3.08 was calculated from 50 healthy controls. A ratio below 2.12 was found in all samples from patients with FVLM (n=21), in 9 of 12 patients with PC, in 0 of 6 with protein S (PS), and in 0 of 4 with antithrombin (AT) deficiency. There was, however, a good discrimination between carriers of the FVLM (highest ratio 1.44) and patients deficient in PC (lowest ratio 1.59), in particular when samples were prediluted with factor V deficient plasma FVDP (1.16 vs. 1.96, respectively). Predilution of samples with FVDP caused a clear discrimination between controls and patients deficient in PC, PS, AT, and FVLM-positive individuals and also in patients on oral anticoagulant treatment. Our data show that the Factor V (Leiden) Test discriminates well between carriers of the FVLM and healthy controls or patients deficient in PC, PS, and AT. Individuals presenting values between the lower cutoff of controls and the range in which FVLM-positive individuals are found are highly suspicious for protein C deficiency.