Normal blood clot formation requires activation of many clotting factors including factor VIII and factor V. Protein C, a natural anticoagulant in blood, when activated (APC) inactivates activated factor V (FVa) and activated factor VIII (FVIIIa) and slows the process of clot formation. In 1993 Dahlback and colleagues reported that when APC was added to the plasma of some patients with unexplained thrombophilia, the expected prolongation of APTT (activated partial thromboplastin time) didn’t occur. Similar abnormality was also found in some family members of these patients. They named this abnormality “Activated protein C resistance (APC-R).” They assumed that this hereditary condition was due to deficiency of yet unrecognized protein C cofactor.

By 1994 it was demonstrated that in about 95% of those with hereditary APC-R, the abnormality was due to a mutation in factor V. This abnormal factor V was named factor V Leiden (FVL). Activated FVL (FVLa) is inactivated by APC at approximately one tenth the rate of inactivation of FVa. FVL is an autosomal dominant condition which exhibits incomplete dominance. Patients who are heterozygous for FVL have a 5- to10-fold and homozygous 50- to 100-fold life-long risk of experiencing a thrombotic disorder. Not all APC-R syndromes are hereditary. Some are acquired. Pregnancy, use of oral contraceptives, high factor VIII and high fibrinogen are associated with acquired APC-R. No anticoagulation is needed in the presence of APC-R, whether hereditary or acquired, unless the patient has actually developed a thrombotic complication.

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