Difference in absolute risk of venous and arterial thrombosis between familial protein S deficiency type I and type III. Results from a family cohort study to assess the clinical impact of a laboratory test-based classification.

Hereditary protein S (PS) deficiency type I is an established risk factor for venous thromboembolism. Contradictionary data on type III deficiency suggests a difference in risk between both types. We studied 156 first degree relatives (90% of eligible relatives) from type I deficient probands (cohort 1) and 268 (88%) from type III deficient probands (cohort 2) to determine the absolute risk of venous and arterial thromboembolism. Annual incidences of venous thromboembolism were 1.47 and 0.17 per 100 person-years in deficient and non-deficient relatives in cohort 1 [relative risk (RR) 8.9; 95% confidence interval (CI) 2.6-30.0], and 0.27 vs. 0.24 in cohort 2 (RR 0.9; 95% CI 0.4-2.2). Type III deficiency was demonstrated in 20% of non-deficient relatives in cohort 1 and the annual incidence in this subgroup was 0.70 (RR 4.3;0.95-19.0). The cut-off level of free PS to identify subjects at risk was 30%, the lower limit of its normal range (65%). PS deficiency was not a risk factor for arterial thromboembolism. In conclusion, type I deficiency was found to be a strong risk factor for venous thromboembolism, in contrast with type III deficiency. This was because of lower free PS levels in type I deficient subjects and a free PS cut-off level far below the lower limit of its normal range.