Antithrombin Deficiency is Associated with Prothrombotic Plasma Fibrin Clot Phenotype.

Background Deficiency of antithrombin increases risk of venous thromboembolism. We hypothesized that antithrombin deficiency affects fibrin clot structure and function. Methods We evaluated 148 patients (age: 38 [32-50] years; 70% women) with genetically confirmed antithrombin deficiency and 50 healthy controls. Fibrin clot permeability (Ks) and clot lysis time (CLT) along with thrombin generation capacity were assessed before and after antithrombin activity normalization in vitro. Results Antithrombin-deficient patients had lower antithrombin activity (-39%) and antigen levels (-23%) compared with controls (both p< 0.01). Prothrombin fragment 1+2 levels were 26.5% higher in patients with antithrombin deficiency than in controls along with 94% increased endogenous thrombin potential (ETP) and 108% higher peak thrombin (all p< 0.01). Antithrombin deficiency was associated with 18% reduced K-s and 35% prolonged CLT (both p< 0.001). Patients with type I (n = 65; 43.9%) compared with type II antithrombin deficiency ( n = 83; 56.1%) had 22.5% lower antithrombin activity (p< 0.001) and despite similar fibrinogen levels, 8.4% reduced Ks, 18% prolonged CLT, and 30% higher ETP (all p< 0.01). Reduced Ks was associated with lower antithrombin antigen level (ss = -6.1, 95% confidence interval [CI]: -1.7 to -10.5), while prolonged CLT was associated with lower antithrombin antigen (ss = -69.6, 95% CI: -9.6 to -129.7), activity (ss = -2.4, 95% CI: -0.3 to -4.5), higher PAI-1 (ss = 12.1, 95% CI: 7.7-16.5), and thrombin-activatable fibrinolysis inhibitor levels (ss = 3.8, 95% CI: 1.9-5.7). Addition of exogenous antithrombin reduced ETP (-42%) and peak thrombin (-21%), and improved Ks (+8%) and CLT (-12%; all p< 0.01). Conclusions Our study suggests that enhanced thrombin generation and prothrombotic plasma fibrin clot phenotype can contribute to increased risk of thrombosis in patients with antithrombin deficiency.