β-Antithrombin, subtype of antithrombin deficiency and the risk of venous thromboembolism in hereditary antithrombin deficiency: A family cohort study

Hereditary antithrombin deficiency is associated with a high incidence of venous thromboembolism (VTE), but VTE risk differs between families. Beta-antithrombin is reported to be the most active isoform of antithrombin in vivo. Whether β-antithrombin activity and subtypes in antithrombin deficiency have impact on VTE risk has not been investigated outside the proband setting. We performed a retrospective family cohort study to investigate whether subtypes of antithrombin deficiency or β-antithrombin levels are associated with the risk of first or recurrent VTE. Eighty-one subjects from 21 families were included, of which 52 were antithrombin deficient. Βeta-antithrombin levels were decreased in most type I and type IIPE subjects, but normal levels were found in all subtypes of antithrombin deficiency. The annual incidence of VTE in antithrombin-deficient family members was 1.24%, 95%CI: 0.72–1.99%, in low β-antithrombin 1.36% (95%CI: 0.76–2.25%) and in normal β-antithrombin 0.79% (95%CI: 0.10–2.77). The annual incidence of recurrence in family members was 3.1% (95%CI: 0.9–7.1%). Duration of anticoagulation had an impact on recurrence risk: In family members annual recurrence with fixed duration was 10% (95%CI: 2.1–29.2%), with indefinite duration 1.5% (95%CI: 0.2–5.4%), p