Factor V Leiden (FVR506Q) is a genetic defect in the factor V (FV) molecule that confers resistance to proteolysis by activated protein C (APC) and is the most common abnormality detected in patients studied for hereditary thrombophilia. The initial screening test for this abnormality was a comparison of the activated partial thromboplastin time (APTT) in the presence and absence of APC, expressed as a ratio. But this has been shown to lack sensitivity for the FV mutation. Other clot-based screening tests, such as the modified APTT, using FV-deficient plasma, or the Russell viper venom (RVV) time assay have improved sensitivity.
Eighty-seven samples were studied using the RVV-based assay. This assay was performed on platelet-poor plasma (PPP-RVV) and whole blood (WB-RVV). All samples were analyzed by polymerase chain reaction (PCR) for the FV Leiden defect: 77 were PCR negative; 10 were PCR positive. Using a threshold ratio of 1.8, all samples were correctly categorized in the PPP-RVV and the WB-RVV tests, showing an observed sensitivity and specificity of 1.0. These results suggest that an RVV-based assay using whole blood could be an effective screening test for this common abnormality.