PROTHROBIN

A coagulation factor needed for the normal clotting of blood. In the cascade of events leading to the final clot, prothrombin precedes thrombin (and so is a precursor to thrombin). Also called thrombinogen. Prothrombin time: A clotting test, a test done to test the integrity of part of the clotting scheme. Familiarly called the "pro time," the test is the time needed for clot formation after a substance called thromboplastin (+ calcium) has been added to plasma.