Thrombin is an enzyme in the blood, which, together with fibrinogen, helps blood clotting. Thrombin time indicates fibrinogen activity. During the blood clotting process, thrombin converts fibrinogen to form fibrin fibres that form a fibrin scaffold. The fibrin scaffold, along with the adherent platelets, creates a blood clot that prevents further blood loss. Thrombin time characterizes the stage in the blood clotting process when fibrin fibres are formed from dissolved fibrinogen.