Conflicting data have been reported on the risk for venous thrombosis in subjects with low free protein S levels. We performed a post-hoc analysis in a single-center retrospective thrombophilic family cohort, to define the optimal free protein S level that can identify subjects at risk for venous thrombosis. Relatives (1143) were analyzed. Relatives with venous thrombosis (mean age 39 years) had lower free protein S levels than relatives without venous thrombosis (P < .001), which was most pronounced in the lowest quartile. Only relatives with free protein S levels less than the 5th percentile (< 41 IU/dL) or less than the 2.5th percentile (< 33 IU/dL) were at higher risk of first venous thrombosis compared with the upper quartile (> 91 IU/dL); annual incidence 1.20% (95% confidence interval [CI], 0.72–1.87) and 1.81% (95% CI, 1.01–2.99), respectively; adjusted hazard ratios 5.6, (95% CI, 2.7–11.5) and 11.3 (95% CI, 5.4–23.6). Recurrence rates were 12.12% (95 CI, 5.23–23.88) and 12.73% (95% CI, 5.12–26.22) per year; adjusted hazard ratios were 3.0 (95% CI, 1.03–8.5) and 3.4 (95% CI, 1.1–10.3). In conclusion, free protein S level can identify young subjects at risk for venous thrombosis in thrombophilic families, although the cutoff level lies far below the normal range in healthy volunteers.

Hereditary protein S (PS) deficiency predisposes to venous thrombosis. Previously, we demonstrated a difference in risk of venous thrombosis between PS deficiency type I and type III. We used direct sequencing, multiplex ligation-dependent probe amplification (MLPA), and linkage analysis to study whether this difference could be explained by molecular heterogeneity. The study contained two sets of families with PS deficiency type I (cohort 1; 35 probands, 155 relatives) or type III (cohort 2; 52 probands, 241 relatives). In cohort 1, a mixed type I/type III PS-deficient phenotype was observed in 66% of the pedigrees. A total of 34 probands carried a mutant PROS1 allele, compared to one proband in cohort 2 (P<10⁻¹⁰). The proband's mutation was identified in all type I, but only in 57% of type III PS deficient relatives. MLPA-analysis in the mutation negative families did not reveal PROS1 deletions or insertions. Linkage analysis in 16 families showed cosegregation of PROS1 markers in the family with type I deficiency, but not in the 15 families with type III deficiency. The genotype–phenotype associations point to differences in genetic architecture. Whereas PS deficiency type I is a monogenic disease due to PROS1 allelic heterozygosity, PS deficiency type III is most likely a more complex or heterogeneous disorder. Hum Mutat 29(7), 939–947, 2008.

Summary: Conflicting data have been reported on the accuracy of protein S (PS) assays for detection of hereditary PS deficiency. In this study we assessed the diagnostic performance of two total PS antigen assays, four free PS assays and three PS activity assays in a group of 28 heterozygous carriers of mutations in PROS1 and 165 control subjects. Several control groups were formed, one of healthy volunteers and – because PS levels are influenced by oral contraception and pregnancy, and assays measuring PS activity may be influenced by the presence of the factor V Leiden mutation -, we also investigated the influences of these factors. All nine PS assays detected significantly reduced PS levels in subjects with a PROS1 mutation. Eight out of nine PS assays showed a 100% sensitivity and 100% specificity to detect heterozygous carriers of mutations in PROS1 with values far below the lower limit of the reference values obtained from healthy volunteers. Low specificities were found in subjects with a factor V Leiden mutation and in pregnant women. At lower cutoff levels, equal to the highest PS value found in heterozygous carriers of mutations in PROS1, the specificity considerably increased in these subjects. When using low cut-off levels equal to the highest PS value found in heterozygous carriers of mutations in PROS1, ensuring 100% sensitivity, the specificity in all study groups increases considerably, by which misclassification can be maximally avoided.