Laboratory testing for activated protein C resistance (APCR): An update

Activated protein C resistance (APCR) reflects a hemostatic state defined by a reduced ability of activated protein C (APC) to affect an anticoagulant response. This state of hemostatic imbalance is characterized by a heightened risk of venous thromboembolism. Protein C is an endogenous anticoagulant that is produced by the hepatocytes and undergoes proteolysis-mediated activation to APC. APC in turn degrades activated Factors V and VIII. APCR describes a state of resistance by activated Factors V and VIII to APC-mediated cleavage of these factors, thereby promoting amplified thrombin production and a potentially procoagulant state. This resistance of APC may be inherited or acquired. Mutations in Factor V are responsible for the most frequent form hereditary APCR. The predominant mutation, a G1691A missense mutation at Arginine 506, the so-called Factor V Leiden [FVL], causes a deletion of an APC-targeted cleavage site in Factor Va, thereby rendering it resistant to inactivation by APC. There are a variety of laboratory assays for APCR, but this chapter focuses on a procedure using a commercially available clotting assay that utilizes a snake venom and ACL TOP analyzers.