W ITH increasing use of tests of the prothrombin concentration as an aid in the diagnosis of diseases of the liver and as a guide in vitamin K and dicumarol therapy,’6 it is imperative that the determinations be accurate. There are many ways of measuring the prothrombin concentration and each admits of numerous sources of error. Even when the most careful technic is practiced, appreciable error...

In contrast to earlier assays, the prothrombin determination described here uses a two-step reaction: first, prothrombin is activated with human factor Xa, phospholipids, Ca++ and factor V; second, the amount of activated prothrombin is assayed by the proteolysis of added fibrinogen. The assay is dependent only on prothrombin, and independent of other components of thrombin generation. Carboxy-...