Free protein S deficiency is a risk factor for venous thrombosis.

A recent study suggests that protein S deficiency is not a risk factor for venous thrombosis. Since this unexpected finding would have important clinical implications if confirmed, we performed a case-control study with the aim to determine the prevalence of protein S deficiency in patients with thrombosis and in healthy individuals taken from the general population and the relative risk of thrombosis in protein S-deficient patients. Free protein S concentration was measured in 327 consecutive patients with at least one venous thrombotic episode and in 317 age- and sex-matched control individuals. Different normal reference ranges were obtained and adopted for men and women. Protein S deficiency was found in 3.1% (95% CI: 1.5-5.2) of patients and in 1.3% of controls (95% CI: 0.3-2.8). Ten patients and 4 control subjects had protein S deficiency, which determined a relative risk of thrombosis (sex- and age-adjusted odds ratio) of 2.4 (95% CI: 0.8-7.9). When men and women were analyzed separately, the risk was 5.0 (95% CI: 0.6-43.6) and 1.6 (95% CI: 0.4-6.7) respectively. PS-deficient men had more thrombotic episodes than women and later in life. Multivariate analysis established that sex was an independent determinant of the number of episodes, as was age, while PS deficiency was not. However sex and PS deficiency status were both determinants of age at first thrombotic episode.