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Coagulation factor, or clotting factor /, one of 13 elements in the blood that help to form blood clots. If normal amounts of calcium, platelets, and tissue factors are present, a chain reaction of clotting factors (prothrombin, thrombin, and fibrinogen) produce a substance called fibrin. Fibrin forms a mesh over the wound in which all of the formed elements are kept rigid. Factors VIII and IX are given intravenously to treat hemophilia. Until recently these were entirely plasma derived, but now recombinant forms of these factors are available.