Four hundred and ninety-three consecutive patients referred for arterial or
venous thrombosis were screened for congenital and acquired abnormalities of blood coagulation predisposing to
thrombosis, and were compared to 341 age- and sex-matched controls. The aim of the study was to determine the prevalence and clinical characteristics of resistance to activated
protein C (APC), a defect shown to have different prevalences in different ethnic groups and to be associated with an increased risk of
thrombosis. Seventy-three (15%) patients had both
APC resistance and the 1691 G to
A factor V gene mutation, compared to 6/341 (2%) controls. Seven patients had
antithrombin deficiency (1.4%), 11 had
protein C deficiency (2.2%), and 4 had
protein S deficiency (0.8%). The relative risk of
thrombosis in APC-resistant patients was 9.4. Resistance to APC was associated mainly with
venous thrombosis, the most frequent being
deep-vein thrombosis of the lower limbs. Fifty-eight percent of APC-resistant patients had an associated risk factor at the first thrombotic event: pregnancy and
oral contraceptive intake were associated with the first thrombotic episode in 35% and 30% of women, respectively.
APC resistance is the most frequent defect of blood coagulation in the general population and in the unselected thrombotic population studied by us.