PLASMA-LEVELS OF FACTOR-II, FACTOR-VII AND FACTOR-X AND THEIR RELATIONSHIP TO THE INTERNATIONAL NORMALIZED RATIO DURING CHRONIC WARFARIN THERAPY

Monitoring of oral anticoagulant therapy is usually undertaken with th e prothrombin time (PT), which is influenced Ly factors II, X, and VII . A number of studies have suggested that the prothrombin (factor II) level may be the most important determinant of the therapeutic efficac y of these drugs. Although some studies suggest that oral anticoagulan ts induce a similar residual level of plasma vitamin K-dependent prote ins, others have called this into question. We therefore measured plas ma levels of factors II, X, and VII in 50 patients undergoing chronic Warfarin therapy. The plasma levels of factors II, X, and VII were sig nificantly different. Although the factor X levels of all plasmas mere < 30%, levels of factors II and VII were > 30% in 14% and 50% of the samples, respectively. Multivariable analysis showed factor II levels to be the least significant of the three factors measured in determini ng the international normalized ratio of plasma or whole blood. Thus, plasma levels of the vitamin K-dependent coagulation factors are not e qual in patients on chronic Warfarin therapy. If factor II (prothrombi n) levels are indeed the major determinants of the therapeutic efficac y of Warfarin, alternative means of monitoring that more accurately re flects prothrombin levels should be evaluated.