The first hypothesis is supported by findings that elevated fibrinogen concentrations were associated with increased thrombus formation and blood viscosity in PH patients15. Also, when obtained from CTEPH patients, fibrinogen is known to be relatively resistant to in vitro proteolysis by plasmin16 and a specific fibrinogen Aα polymorphism increased the risk of developing CTEPH by strengthening the fibrinogen chain cross-linkage17. Fibrinogen's pathological impact on coagulation in CTEPH was further substantiated by a high prevalence of 5 different dysfibrinogenaemias (fibrinogenSan Diego I-V) among patients with CTEPH. All 5 dysfibrinogenaemias seem to confer resistance to fibrinolysis thereby fostering CTEPH development18.