The estimation of prothrombin in human plasma

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Abstract
Plasma, in 1:50 to 1:800 dilution, was incubated with optimal amts. of Ca and thrombo-plastin, and the thrombin formed was estimated from the time required to form a clot with added fibrinogen. The quantitative aspects of the various reactions involved were studied. Expts. with separated thrombin preps. showed that the clotting time with added fibrinogen was inversely proportional to the conc. of thrombin. With separated prothrombin preps., the thrombin formed was directly proportional to the prothrombin present. With diluted plasma, there was some loss of thrombin, attributed to the action of plasma antithrombin, which was appreciable in the range of dilutions used. The proportion of thrombin lost was regularly related to the plasma dilution; a geometric series of plasma dilutions with the factor 1/2 corresponded to a geometric series of clotting times with the factor 1/1.6. This empirical formula applied both to normal plasma and to the prothrombin-deficient plasma of obstructive jaundice. The titres of an abnormal and a standard normal plasma could be compared on the basis of the ratio of the clotting times with added fibrinogen at any given dilution of plasma. The method gave a reasonably quantitative scale of titres; the range of normal variation was from 85-116% of the normal avg. titre.

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