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Coagulation Factor V (FV) is a key factor for regulating blood coagulation cascade, and it acts at the crossroads of the intrinsic and extrinsic pathways. It shows a dual activity as the procoagulant cofactor for Factor Xa in the prothrombinase complex, but it also supports an anticoagulant activity in combination with TFPI and Protein S. Its rapid cleavage by Activated Protein C (APC) complexed with Free Protein S (FPS), in presence of phospholipids and calcium, inhibits its activity and limits the propagation of blood coagulation, keeping it to where it is beneficial. Rapid inactivation of active FV by APC-FPS is essential for preventing the risk of thrombosis development. In 1993, Dahlbäck and coworkers reported an inherited disorder characterized by activated protein C resistance (APC-R) and associated to an increased occurrence of thromboembolic events in affected families. In 1994 Bertina demonstrated that this diathesis resulted from a Factor V mutation (R506Q), rendering this factor resistant to inactivation by APC. This mutated Factor V was called Factor V Leiden (FV-L). APTT based assays and molecular biology methods for detecting the mutation were developed, but these methods are only qualitative and classify tested individuals as normals, heterozygous or homozygous for the coagulation defect. Our group developed a quantitative assay for FV-L, which is described in this report, along with its performances. This assay allows to quantitate specifically FV-L coagulant activity, and to graduate its amount in heterozygous or homozygous patients. FV-L is absent in normal individuals and present in homozygous or heterozygous patients, accounting respectively for 100 % or 50 % of blood FV. Its amount is compared with FV clotting activity or antigenic concentration. Measured FV-L activities overlap between heterozygous patients with high FV and homozygous ones with low FV levels. This assay allows to better discriminate for the FV-L associated thrombotic risk, which depends on the effective FV-L concentration rather than on patients’ genetic status. This expectation is supported by literature review, which shows that FV-L concentrations correlate with presence of platelet released microparticles in patients carrying that mutation.