Summary. Resistance to activated protein C (APC) is at present considered the most frequent laboratory abnormality in patients with deep-vein thrombosis. An increased risk for venous thrombosis is associated to the use of oral contraceptives (OC).
We studied APC sensitivity in 50 healthy women taking OC and in 50 healthy controls, matched for age, smoking habit, educational and social levels, and the main biochemical routinary parameters. Subjects with a personal or familial history of thrombosis and also with chronic or acute diseases were excluded. Protein C, protein S, antithrombin III and lupus anticoagulant activity (LAC) were also evaluated. Increased fibrinogen and protein C levels, decreased protein S, and shortened PT and APTT were also observed in women taking OC. APC sensitivity ratio (APC-SR) was significantly lower in the OC group than in a control group (2.6 ± 0.38 v 2.81 ±0.35, P<0.01). Seven of eight women with APC ratio ≤ 2 (APC resistant) were OC users: the difference of prevalence was statistically significant (chi-squared test, P<:0′05). Only two out of eight women were found heterozygous for the Leiden factor V mutation. Two APC-resistant women without the Leiden mutation subsequently discontinued OC and both then normalized their APC-SR. We conclude that acquired factors, i.e. oral contraceptives, may play an important role in determining plasma APC resistance.