Ecarin, the primary reagent in this assay, is derived from the venom of the saw-scaled viper, Echis carinatus.

[citation needed] 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.

Thus, the ECT is prolonged in a specific and linear fashion with increasing concentrations of hirudin.

This assay shows no interference from prothrombin or fibrinogen in the sample and is suitable for the measurement of all direct thrombin inhibitors.