Recent advances in the study of blood coagulation have been stimulated by the discovery of vitamin K and its relation to prothrombin. While there is considerable variance of opinion as to the nature of blood coagulation, it can be assumed that at least five factors are necessary before this phenomenon occurs. These include ionizable calcium, thromboplastin, prothrombin, thrombin and fibrinogen. It is generally agreed that these substances act in the following manner, as was suggested by Howell1 in 1914.
Based on this theory of coagulation, several methods for the measurement of prothrombin have been devised. Of these, the two stage method of Warner, Brinkhous and Smith2 and the one stage method of Quick3 have been more frequently employed than the others.
The two stage technic of Warner, Brinkhous and Smith is carried out as follows: Oxalated plasma is defibrinated by the addition of purified thrombin.2 This