Resistance to Activated Protein C as a Basis for Venous Thrombosis

BACKGROUND: In three families with various forms of venous thrombosis, we observed an apparently inherited poor response to the anticoagulant activated protein C (APC). The condition was due to a deficiency in a previously unrecognized anticoagulant factor that functioned as a cofactor to activated protein C. METHODS: We conducted the present study to determine the prevalence of resistance to APC in patients with venous thrombosis. We compared 104 consecutive patients with venous thrombosis confirmed by objective tests with 130 controls. In addition, 211 members of 34 families of persons with resistance to APC were studied. The anticoagulant response to APC was measured with a modified version of the activated partial-thromboplastin time test; the results were expressed as APC ratios. RESULTS: Forty-five percent of patients had a family history of thrombosis. A significant (P < 0.001) difference in APC ratios was observed between the controls and the patients with thrombosis. For 33 percent of patients, the APC ratio was below the 5th percentile of the control values, although the results of the family studies suggested that the prevalence of APC resistance may be even higher (approximately 40 percent) in the patients with thrombosis. The inherited nature of the defect was confirmed in a majority of cases, and the family studies suggested the mode of inheritance to be autosomal dominant. The thrombosis-free survival of APC-resistant family members was significantly less than that of non-APC-resistant family members. CONCLUSIONS: There was a high prevalence of APC resistance among young persons with a history of venous thrombosis, and this resistance appeared to be inherited as an autosomal dominant trait.