Serine protease that selectively cleaves Arg-Gly bonds in fibrinogen to form fibrin and fibrinopeptides A and B.

• Activity is expressed in NIH units obtained by direct comparison to a NIH thrombin reference standard

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 (stabilised in a 1% buffered albumin solution) based on a modification of the method of Biggs. Only clotting times in the range of 15 to 25 seconds are used for determining thrombin concentrations.