Activated protein C resistence screening

Down-regulation of blood coagulation within a forming clot is accomplished by the protein C anticoagulant pathway. The essential co-factors FVa and FVIIIa are degraded through proteolytic cleavage by activated protein C (APC). Protein C is activated by thrombin in complex with thrombomodulin, a transmembrane endothelial cell protein.

Activated protein C resistance (APCR) is defined as a poor anticoagulant response of plasma to APC and has both hereditary and acquired causes. More than 90% of hereditary cases are due to the FV Leiden (FVL) point mutation which results in an impaired inactivation rate of FVa due to alteration of a protein C cleavage site. Native FVa is an important procoagulant co-factor to FXa in the prothrombinase complex and the result of the mutation is that thrombin formation will not be stopped as efficiently as in the case of inactivation of native FVa. Compared to individuals with a normal FV genotype, heterozygosity for the mutation confers about a 5 – 10 fold higher thrombotic risk. Other FV mutations conferring at least in vitro APCR include FV Cambridge and FV Hong Kong. No mutations in the FVIII gene conferring APCR have been described. Acquired APCR has been described in association with pregnancy, high FVIII levels, antiphospholipid antibodies and use of the oral contraceptive pill.