Warfarin monitoring by an anticoagulant therapy factor (ATF)

In a pilot study (1997) using POTENS+, our coagulation instrument, we determined that: (a) an Anticoagulant Therapy Factor (ATF) was comparable to the International Normalized Ratio (INR) for monitoring warfarin anticoagulant therapy, (b) one could use any of the four thromboplastins with which the ATF was derived with comparable results, and (c) the ATF could be proposed to monitor warfarin therapy. The ATF-INR comparisons correlated well statistically; but when individual ATF-INR comparisons were later studied, there were frequent discrepancies. The pilot study (1997) was based on hospitalized patients, so almost all patients were undergoing induction of warfarin anticoagulation. Since none of them had taken warfarin for at least six weeks, none of them could be considered "stable" on warfarin. In the present study, all patients were on warfarin therapy for at least six weeks, and the ATF equation was modified by multiplying it by the prothrombin ratio (PR) to give a corrected ATF (CATF). This CATF was then further modified to achieve agreement with the INR by adjusting the linear regression line by means of analytic geometry, so that the CATF-INR regression line now had a slope of one and passed through the origin. With these changes, the modified ATFs (MATF) and INRs correlated well and were nearly equal numerically when using two of the four thromboplastins. Reason for the discrepancies with the other two thromboplastins will be discussed.