Activated protein C resistance is a hereditary blood disorder that causes abnormal venous thrombosis. Treatment involves heparin injections followed by oral warfarin therapy. The disorder is caused by a mutation in coagulation factor V, which makes it immune to activated protein C. The risk of developing thrombosis is increased in people with inherited activated protein C resistance, and biological stressors can trigger the disorder. Acquired resistance can also occur due to cancer, inflammatory diseases, lupus, high cholesterol, oral contraceptive use, and pregnancy. Testing involves prothrombin time and activated partial thromboplastin times.
Activated protein C resistance refers to a blood or hemostasis disorder that occurs when the enzyme fails to regulate blood clotting, resulting in abnormal venous thrombosis. Over 80% of the time, the condition is hereditary. Once diagnosed, treatment involves injections of heparin followed by lifelong oral warfarin therapy.
Protein C normally becomes activated when exposed to thrombin combined with thrombomodulin on endothelial cells, or the lining, of blood vessels. Upon activation, protein C typically breaks down the clotting factors Va and VIIIa, preventing blood clots from forming. Researchers have discovered that the inherited disorder involves a mutation in coagulation factor V. This mutation, called factor V Leiden, makes this particular clotting factor immune or resistant to the effects of activated protein C, negating an anticoagulant response.
Scientists suggest that the risk of developing thrombosis is nearly eight times greater in people with inherited activated protein C resistance than in the average person. The risk generally increases dramatically up to 90 times greater for those who inherit identical pairs of the gene. Deep vein thrombosis (DVT) is a common symptom, but blood clots can form anywhere along the venous system. Abnormal blood clot formation may appear as early as adolescence, and biological stressors, including infections, pregnancy, or surgery, are usually contributing factors.
Cancer, inflammatory diseases, and lupus can lead to acquired resistance to activated protein C. High cholesterol, oral contraceptive use, and pregnancy could also trigger the disorder. In acquired disease, there is no coagulation factor mutation. In the case of lupus, chemical reactions that occur in the body cause resistance to factor V without mutation. In the other conditions, researchers believe that resistance to activated protein C occurs because the body has abnormally high levels of coagulation factors VII and VIII and, possibly, higher levels of fibrinogen. Without sufficient amounts of activated protein C, these elevated clotting factors cause blood clots to precipitate.
When doctors suspect activated protein C resistance, they can do prothrombin time (PT) and activated partial thromboplastin times (aPTT). The PT and aPTT relate to the time it takes for blood to clot normally and after the addition of calcium and a phospholipid. In the absence of possible infection or autoimmune disease, activated protein C resistance could be added to the plasma to evaluate drug clotting time versus normal PT times. Similar times between the two tests could be indicative of the disorder. Additional testing may include identification of the factor V Leiden mutation.
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