The Assay Recovery of Prothrombin Added to Plasma

Abstract

In the promulgation of current methods of prothrombin assay 1 , 2 there has been some investigation of plasma dilutions but no direct experiments to show the ability to recover prothrombin added to plasma. This is important in connection with the possibility that clot-inhibitors 3 , 4 , 5 , 6 may complicate the interpretation of the assay. It is recognized, for instance, that heparin interferes with prothrombin determinations 2 and heparin can act both as an antiprothrombin 6 and as an immediate antithrombin, 5 in the presence of the respective plasma co-factors. The natural antithrombins also include a progressive thrombinolytic factor. 4 As an example of the kind of interference which may be expected, it has recently been found that a series of prothrombin dilutions, subsequently activated with Ca⁺⁺ and brain thromboplastin, gives more than the theoretical thrombin yield, as determined from the clotting-times for purified fibrinogen, using a similar dilution series of the same (full-strength) thrombin as the standard of reference. 6 In the absence of equivalence it is, of course, impossible to assay prothrombin both in terms of an arbitrarily fixed clotting-time and a definite thrombin dilution value. Inherent in the clinical tests is a false assumption that inhibitors do not influence the results obtained. Actually, there must be a rôle for the natural inhibitors of the types mentioned. The Iowa workers 2 recognized only the progressive antithrombins and succeeded in controlling these by dilution. It is an unproved assumption that immediate antithrombins and antiprothrombins do not interfere with the prothrombin assay. In the following experiment, in which a prepared prothrombin is assayed in the presence of plasma, these problems are readily brought to test.