The Factor V Leiden (FVL) mutation is the most common heritable, prothrombotic risk factor in the United States. FVL increases the risk of thromboembolism 4-7 times that of normal in heterozygous individuals, and 40-80 times normal in homozygotes. Testing for this mutation is thus very useful in patients who present with thrombi as it can uncover the reason for their clotting, inform their risk of future clotting, and guide their anti-coagulant management.
There are a few different ways to test for APC resistance. Russell's viper venom time (RVVT) is a screening test that can be used for FVL patients. In this test, the venom directly activates factors V and X. If the patient has the FVL mutation their clotting time is decreased (since the mutation promotes clotting). The plasma is run in duplicate: once in the presence of APC and another in which APC is not added. A ratio between the two times is determined and the results reveal whether the patient's APC is functional or not. However, the APC-resistance ratio may be indeterminate for patients who have a lupus anticoagulant (an antibody to phospholipids that interferes with coagulation tests), or patients who have high heparin levels in their plasma.
RVVT and clotting-time-based screening tests are now less common since a much simpler and cheaper genetic test can be performed. A simple restriction endonuclease PCR test can reveal the FVL mutation.
