Process for determining hirudin and synthetic thrombin inhibitors

A process for determining a concentration of hirudin or of a synthetic thrombin inhibitor selected from the group consisting of a derivative of a tripeptide phe-pro-arg and a hirolog in blood, in a blood component or in a body fluid other than blood, comprising adding a material selected from the group consisting of (1) a prothrombin intermediate selected from the group consisting of meizothrombin, PIVKA meizothrombin, PIVKA being an abbreviation for protein induced by vitamin K antagonist, and meizothrombin-des-fragment-1 or a pharmaceutically acceptable salt of said prothrombin intermediate, (2) a compound that splits prothrombin into meizothrombin or a pharmaceutically acceptable salt of said compound and (3) a mixture of said compound that splits prothrombin into meizothrombin or a salt thereof and said prothrombin intermediate or a salt thereof, to the blood, blood component, or a body fluid other than blood, measuring an elapsed time between said adding and the start of coagulation in said blood, blood component or a body fluid other than blood and comparing the elapsed time measurement with a standard in order to determine said concentration.