Description_x000D_ Packaging_x000D_ 10000, 100000, 250000, 750000 units in glass bottle_x000D_ Biochem/physiol Actions_x000D_ Serine protease that selectively cleaves Arg-Gly bonds in fibrinogen to form fibrin and fibrinopeptides A and B._x000D_ Unit Definition_x000D_ Activity is expressed in NIH units obtained by direct comparison to a NIH thrombin reference standard._x000D_ Analysis Note_x000D_ Activity is expressed in NIH units obtained by direct comparison to a NIH Thrombin Reference Standard, Lot K._x000D_ The NIH assay procedure uses 0.2 mL of diluted plasma (1:1 with saline) as a substrate and 0.1 mL of thrombin sample (stabilized in a 1% buffered albumin solution) based on a modification of the method of Biggs. Only clotting times in the range of 15-25 seconds are used for determining thrombin concentrations.