Anticoagulant protein activity value thresholds for the occurrence of venous thromboembolism in pregnant women with thrombophilia.

This study estimated anticoagulant protein activity value thresholds for the occurrence of venous thromboembolism (VTE) in pregnant women with thrombophilia using national surveillance data. Activity values at diagnosis of thrombophilia were obtained in 77 patients with antithrombin (AT) deficiency, 40 with protein C (PC) deficiency, and 237 with protein S (PS) deficiency. Patients were classified into 11 groups by the occurrence of VTE associated with the current pregnancy, and by family and/or past history of VTE. The median activity values in all patients were 45.8% for AT, 51.0% for PC, and 41.0% for PS. Neither AT nor PC activity differed significantly across the evaluated groups. However, PS activity was lower in the group of all patients with VTE than in the group with no VTE and no family or past history of VTE (34.9 ± 11.8% vs. 44.5 ± 12.6%), indicating an association between lower PS activity and VTE occurrence. ROC-based optimization identified PS ≤ 43% as the most discriminative cutoff value, with a higher estimated risk ratio of 4.02. In contrast, a clinically plausible threshold of PS ≤ 35% yielded a risk ratio of 3.34. Our data may be helpful in determining whether to provide peripartum thromboprophylaxis in pregnant women with thrombophilia.