Background COVID‐19 is associated with a substantial risk of venous thrombotic events, even in the presence of adequate thromboprophylactic therapy. Objectives We aimed to better characterize the hypercoagulable state of COVID‐19 patients in patients receiving anticoagulant therapy. Methods We took plasma samples of 23 patients with COVID‐19 who were on prophylactic or intensified anticoagulant therapy. Twenty healthy volunteers were included to establish reference ranges. Results COVID‐19 patients had a mildly prolonged prothrombin time, high von Willebrand factor levels and low ADAMTS13 activity. Most rotational thromboelastometry parameters were normal, with a hypercoagulable maximum clot firmness in part of the patients. Despite detectable anti‐activated factor X activity in the majority of patients, ex vivo thrombin generation was normal, and in vivo thrombin generation elevated as evidenced by elevated levels of thrombin‐antithrombin complexes and D‐dimers. Plasma levels of activated factor VII were lower in patients, and levels of the platelet activation marker soluble CD40 ligand were similar in patients and controls. Plasmin‐antiplasmin complex levels were also increased in patients despite an in vitro hypofibrinolytic profile. Conclusions COVID‐19 patients are characterized by normal in vitro thrombin generation and enhanced clot formation and decreased fibrinolytic potential despite the presence of heparin in the sample. Anticoagulated COVID‐19 patients have persistent in vivo activation of coagulation and fibrinolysis, but no evidence of excessive platelet activation. Ongoing activation of coagulation despite normal to intensified anticoagulant therapy indicates studies on alternative antithrombotic strategies are urgently required.
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