Free and Total Protein S Antigen Levels on the Risk of Venous Thrombosis: Results From the MEGA Study

538 Context: Conflicting data have been reported on the risk for venous thrombosis in individuals with low total protein S levels (i.e. type I protein S deficiency) and low free protein S levels with normal total protein S levels (i.e. type III protein S deficiency). This may be due to small numbers, wrong cut off level or inclusion of individuals with mild transitory decrements in protein S levels. Most studies that showed that type I and type III protein S deficiency were related with an increased risk for venous thrombosis, have been performed in thrombophilic families, suggesting that these deficiencies are inherited. As the prevalence of inherited type I or type III protein S deficiency is not known, the relevance of these findings within normal populations remain to be established. Objectives: To assess the risk of first venous thrombosis in persons with low levels of free protein S or total protein S in a large population-based case–control study. Design: MEGA study, 4956 consecutive patients aged 18 to 70 years with a first episode of venous thrombosis were included. Age- and sex-matched controls were partners of patients (n=3297) or individuals recruited by random digit dialing (n=3000). DNA was obtained by standard methods and was available for 4485 patients and 4889 control subjects. Citrated plasma was available for 2471 patients and 2940 controls. Molecular basis for protein S deficiency was investigated by analysis of copy number variation of PROS1 and sequencing of individuals with the lowest levels of protein S in attempt to explain the different findings in risk estimates between families and population studies. Statistical analysis: Odds ratios were adjusted for age and sex (matching factors) for levels of free/total protein S and their 95% confidence levels (95% CIs) with the use of logistic regression. The 2.5th-97.5th percentile of both total and free protein S in control subjects that did not use vitamin K antagonists (VKA) at time of blood draw were considered as the reference range. Individuals that used VKA at time of blood draw were excluded when calculating relative risk estimates. Furthermore, a preplanned sensitivity analysis was performed where we excluded estrogen users and pregnant women at time of blood draw. Results: Individuals with low free protein S levels or low total protein S levels (<2.5th percentile) were not at increased risk of venous thrombosis as compared to individuals with protein S levels in the 2.5th-97.5th percentile; odds ratio 0.82 (95% CI, 0.56–1.21) and 0.90 (95% CI, 0.62–1.31) respectively. Excluding all women who used estrogens or were pregnant/puerperic at time of venous thrombosis or at time of blood sampling increased the odds ratios slightly to 1.55 (95% CI, 0.84–2.88) for individuals with low free protein S levels and to 1.28 (95% CI, 0.70–2.35) in individuals with low total protein S levels. We subsequently compared decreasing cut off values of free and total protein S levels on the risk of venous thrombosis as compared to the same reference group. Although numbers became small, it appeared that a free protein S cut off level of < 0.20th or < 0.10th percentile could identify individuals at high risk of venous thrombosis (odds ratios 2.01; 95% CI, 0.57–7.15, and 5.44; 95% CI, 0.61–48.78, respectively). Even extremely low (<0.10th percentile) total protein S levels were not associated with venous thrombosis. Only one patient had a copy number variation of PROS1 in 2270 consecutive samples tested. Currently, we are sequencing the PROS1 gene in all individuals with protein S levels <1st percentile of which results will be available before the ASH conference of 2011. Conclusion: Low free protein S and low total protein S levels could not identify individuals at risk for venous thrombosis in a population based study. Although extremely low free protein S levels were associated with an increased risk for venous thrombosis, numbers were too small to support testing on free protein S in an unselected group of venous thrombosis patients. Disclosures: No relevant conflicts of interest to declare.