An APTT-based kit for the screening of factor-V-related APC resistance. The high sensitivity and specificity of the test for the factor V:Q506 mutation is obtained by prediluting the sample plasma with an excess of V-DEF Plasma bioreagent. The test design makes it possible to discriminate between heterozygous and homozygous factor V genotypes. It also allows for analysis of plasma from patients on heparin or oral anticoagulant therapy. High discrimination between genotypes with 100% sensitivity for FV:Q506. Reduces need for PCR determination. Applicable to anticoagulant treated patients.