RELIABILITY OF THE INTERNATIONAL NORMALIZED RATIO FOR MONITORING THE INDUCTION-PHASE OF WARFARIN - COMPARISON WITH THE PROTHROMBIN TIME RATIO

The lnternational Normalized Ratio (INR) was introduced to reduce the variability of prothrombin time (PT) reporting. One potential problem with the use of the INR is the assumption that its reliability is redu ced when it is used to monitor patients during the induction phase of treatment. This shortcoming arises because the model used to establish the INR system is based on the use of pooled plasma from patients sta bilized on warfarin for at least 6 weeks. Because the prolongation of the PT by warfarin during the induction phase mainly reflects reductio n in factor VII levels (whereas the prolongation of the PT after 6 wee ks of stabilization reflects reductions in factors X, II, and VII), th ere exists a potential for loss of accuracy of the INR during warfarin induction. To overcome this potential problem, it has been suggested that the PT ratio should be used to report results during the inductio n phase of treatment and that the INR system should be reserved for re porting results after the patient has been stabilized. This approach i s confusing to the clinician. In addition, the validity of this approa ch has never been demonstrated in a clinical study. To address this is sue, we studied 43 patients for the first 5 days offer they started wa rfarin therapy. We measured the PT in the same plasma samples from eac h patient with five different commercial thromboplastins. The variance in the PT ratios among the five thromboplastins was compared with the variance obtained with the INR values derived from the PT ratios when using the international sensitivity indexes provided by the manufactu rer. Our results indicate that, even during the induction phase, there is less variance with the INR system than with the PT ratio system.