Variations in the reaction between thrombin and fibrinogen and their effect on the prothrombin time.

The appearance of fibrin provides the end-point in every clotting test, and the duration of the thrombin-fibrinogen reaction is thus a component of any measurement of clottinig time' While many tests are designed to study the early stages of clotting, in interpreting the results it is necessary to consider to what extent the observed differences may be due simply to differences in the " indicator reaction" between thrombin and fibrinogen. This precaution particularly applies to one-stage tests in which comparisons are made between different bloods. The reaction between thrombin and fibrinogen may be separately investigated, and fortunately the measurement of the thrombin clotting time of citrated plasma (Biggs and Macfarlane, 1953) is one of the simplest of all clotting tests. This paper reports a case in which the one-stage prothrombin time was spuriously lengthened by an abnormally slow thrombin-fibrinogen reaction; tests were then made in four other cases to investigate the processe3 involved.