In US: 510(k) Cleared for In Vitro Diagnostic Use. Clotting method for the quantitative measurement of Factor V-L (Factor V Leiden) concentration in citrated plasma, by its resistance to the action of Activated Protein C (APC). The assay is performed in the presence of Activated Protein C and Protein S (one single test for each patient). In the presence of APC, the prolongation of clotting time is an inverse relationship of the concentration of Factor V-Leiden (mutation R506Q). Normal Factor V is not measured.