Venous thrombosis: factor V G1691A genotyping related to APC resistance as measured by 2 methods.

Blood samples were collected from consecutive unrelated patients with venous thrombosis. The patients originate from the middle part of Sweden. We investigated the presence of the reported point mutation at nt 1691 of factor V which renders the protein resistant to cleavage by activated protein C (APC). Thirty-seven per cent of the patients were heterozygote carriers, and 4.5% were homozygotes for a mutated factor V gene. In addition, resistance to coagulation induced by APC was measured, both by the conventional APTT assay and by a modified APTT assay which has been reported to have an increased resolution. Compared to a control group of healthy people, the mean value was significantly lower in the patient group. A strong correlation between low APC ratio and presence of the factor V mutation was found. By using the modified method, a complete resolution of carriers of the factor V mutation and people with normal factor V alleles was found. However, there was still an overlap between heterozygote carriers and people homozygous for the mutation. The modified method was also found useful in patients treated with warfarin. Among 40 healthy blood donors 7% were found to be heterozygous.