Cleavage of this site by APC is necessary for exposure of the two additional cleavage sites needed for inactivation. The presence of the FVL allele is the major cause of APCr and a well-documented risk factor for venous thrombosis. In a normal patient, the APTT is prolonged in the presence of APC due to the anticoagulant action of this protein. Samples from patients with FVL will usually prolong the clotting time resulting in resistance to APC . A second generation, modification of the original assay is currently the method of choice.
Patient plasma is diluted with factor V-deficient plasma.
This limits variations in plasma handling, minimizes the effects of oral anticoagulant therapy, and improves the sensitivity and specificity for the Leiden allele to 100 percent, while making it cost efficient. If a patient has a normal result with this assay, one does not need to proceed with costly DNA testing for the Leiden mutation. The penetrence of clinical symptoms, i.e., thrombosis, preeclampsia, placental abruption, intrauterine growth retardation and stillbirth, is highly variable in APC resistant individuals. Some never get thrombosis, whereas others suffer from recurrent severe thrombotic events. Homozygosity for the Leiden mutation gives a much higher risk of thrombosis than heterozygosity.