Abstract
The individual importance of each of the four vitamin Independent clotting factors
on the generation of prothrombinase activity in the plasma of orally anticoagulated
patients has been investigated.
Addition of purified factors VII, IX or X to plasma from deeply anticoagulated patients
(International Normalized Ratio 2.8-4.8) did not influence the amount of prothrombinase activity or the amount of thrombin formed.
Only the prothrombin level in the plasma determines the course of thrombin generation.
Addition of increasing amounts of purified factor II, VII, IX or X to plasmas deficient
in respectively factor II, VII, IX or X showed that the prothrombinase activity increases
linearily with the concentration of factor II added and that the concentration below
which the factors VII, IX and X start to have a measurable effect on prothrombinase
activity are 5%, 20% and 30%, respectively. Half maximal prothrombinase activity was
found at about 1% factor VII, 5% factor IX and 8% factor X respectively.
From these observations we conclude that primarily the variation in factor II level
determines thrombin generation and hence presumably the antithrombotic effect of oral
anticoagulant therapy. It therefore seems likely that, for the control of oral anticoagulant
therapy, tests that reflect factor II activity would be suitable.
Keywords
Anticoagulant treatment - Prothrombin - Prothrombinase - Vitamin K antagonists
