What is Activated Protein C-Resistance Ratio?
Activated protein C is an endogenous anticoagulant serine protease that, together with its cofactor protein S, inactivates factors Va and VIIIa, thereby down-regulating thrombin generation. The APCR test measures the prolongation of clotting time when exogenous APC is added to patient plasma; in normal plasma the clotting time is substantially prolonged, yielding a high ratio. The most common cause of resistance is the Factor V Leiden mutation (G1691A), which renders factor Va resistant to APC-mediated cleavage; acquired causes include pregnancy, oral contraceptive use, and high factor VIII.