Predictive value for increased activated factor XI activity in acute venous thromboembolism

Background: Venous thromboembolism (VTE) is associated with excessive coagulation activity, which in part can be attributed to activation of contact system. However, the knowledge regarding the impact of contact activation in acute VTE is limited.Objective: To unravel the involvement of contact activation in acute VTE. Methods: Contact activation was investigated in patients with acute VTE (n = 321) and population controls without a history of VTE (n = 300). For comparison, Factor XI(a) levels, activity, and plasma kallikrein (PKa) activity were determined in plasma samples with an activated partial thromboplastin time- or thrombin generation-based assay (free FXI concentration [FXI:c] and calibrated automated thrombogram:FXIa, respec-tively) and with enzyme-linked immunosorbent assays for enzyme-inhibitor complexes (FXIa:alpha-1-antitrypsin [& alpha;1AT], FXIa:antithrombin [AT], FXIa:C1-inhibitor [C1Inh], and PKa:C1-inh).Results: In patients with VTE, higher FXI:c levels (124 & PLUSMN; 37% vs 114 & PLUSMN; 28%), but lower calibrated automated thrombogram:FXIa levels were apparent. This was accompanied by increased FXIa:& alpha;1AT, FXIa:AT, and PKa:C1-inh levels in patients compared with controls (312pM [238-424] vs 203pM [144-288]; 29pM [23-38] vs 23pM [20-30]; 1.9nM [1.2-4.7] vs 1.4nM [0.7-3.5], respectively), whereas FXIa:C1-inh levels did not differ. Logistic regression models showed good discriminatory values for FXI:c and FXIa:& alpha;1AT (area under the curve = 0.64 [0.6/0.69] and 0.73 [0.69/0.77], respectively). After a 2-year follow-up, 81 recurrent VTE events or deaths occurred in the patient cohort, for which the baseline levels of FXIa:& alpha;1AT and FXIa:C1Inh had a significant prognostic value (Hazard ratios per SD [95% CI], 1.26 [1.10-1.45]; p =.0012 and 1.19 [1.05-1.36]; p =.0082, respectively). Conclusion: Our study revealed elevated FXIa levels and activity in acute VTE, which was also associated with recurrent VTE, suggesting an important risk contribution of FXI activation to VTE. The evidence provided by this study supports the utility of FXIa inhibition in the setting of acute VTE.