Abstract: The disclosed Factor V Ratio (FVR) screening blood assay (read as "factor five ratio") and kits for the conduct thereof, identify individuals that possess a specific genetic defect, known as the Factor V Leiden defect, or other genetic or acquired Factor V defect, that makes those individuals susceptible to venous thromboembolism. In this test the Factor V activity of a blood plasma sample exposed to activated Protein C (APC) is compared to the Factor V activity of a similar sample in the absence of APC, after both samples had been treated with an activating agent. The ratio between the Factor V activity level without APC and the Factor V activity level with APC, identifies individuals at risk of a thrombotic disorder due to a Factor V defect and differentiates between individuals with a heterozygous defect and individuals with a homozygous defect.