ACTIVATED PROTEIN C RESISTANCE AND FACTOR V LEIDEN MUTATION
Activated protein C (APC) resistance is a common condition that can occur with other hypercoagulable states. APC is an anticoagu-lant, and resistance to APC increases the risk for venous thrombo-sis. A molecular defect in the factor V gene has been identified in most (90%) of those with APC resistance; this defect is called fac-tor V Leiden mutation.
It has been identified as the most common cause of inherited hypercoagulability in Caucasians, but its inci-dence appears to be much lower in other ethnic groups. Factor V Leiden mutation synergistically increases the risk for thrombosis in patients with other risk factors (eg, use of oral contraceptives, hyperhomocystinemia, increased age). It does not appear that the use of postmenopausal hormone therapy in women increases the risk for thrombotic events as does the use of oral contraceptives; the dose of estrogen in the former situation is much lower than in the latter. People who are homozygous for the factor V Leiden mutation are at extremely high risk for thrombosis.