 In the clinical assay, a known quantity of ecarin is added to the plasma of a patient treated with hirudin.Ecarin activates prothrombin through a specific proteolytic cleavage, which produces meizothrombin, a prothrombin-thrombin intermediate which retains the full molecular weight of prothrombin, but possesses a low level of procoagulant enzymatic activity.Crucially, this activity is inhibited by hirudin and other direct thrombin inhibitors, but not by heparin.