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 < or = 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.
PIP: During April-June 1994, at Borgo Roma Polyclinic in Verona, Italy, clinical researchers compared data on 50 healthy women 18-41 who used low-dose combined oral contraceptives (OCs) with data on 50 healthy women matched for age, smoking, education, social class, and biochemical routinary parameters. Almost all the subjects were medical students or medical staff working in the hospital where the study occurred. The researchers aimed to examine the prevalence of resistance to activated protein C (APC) in both groups. They also evaluated protein C, protein S, antithrombin III, and lupus anticoagulant activity. The APC-sensitivity ratio (APC-SR) was much lower in OC users than nonusers (2.6 vs. 2.81; p 0.01). Seven of the eight women with an APC-SR of no greater than 2 (i.e., demonstration of APC resistance) used OCs (p 0.05). Prevalence of APC resistance was higher among OC users than nonusers (14% vs. 2%; p 0.05). Among the eight women with APC resistance, two were heterozygous for the Leiden factor V mutation. One of these women used OCs and the other did not. Two APC resistant women who did not have the Leiden factor V stopped using OCs and their APC-SR subsequently normalized. OC users had higher fibrinogen and protein C levels, a lower protein S level, and shorter prothrombin and activated partial thromboplastin times than nonusers. These findings suggest that OCs may contribute to plasma APC resistance, which in turn increases the risk of venous thrombosis.