Clinical significance of Thrombotest and PT-INR in monitoring the oral anticoagulant therapy

Thrombotest (TT) was widely used for monitoring the oral anticoagulation therapy in Japan. Its therapeutic ranges recommended were between 10 to 25% in Japan. We studied the correlation of Thrombotest Owren (TT) with international normalized ratio of prothrombin time (PT-INR). PT-INR was determined from the single regression equation, PT-INR=1.0420×TT-INR-0.0987 (r=0.905, P<0.001) which was obatained from single TT and PT measurements in 1, 157 patients on chronic warfarin therapy.A correlation between the incidence of thromboembolism (TE) and TT and PT values were determined in 170 patients with prosthetic heart valves and on warfarin therapy immediately following surgeries.An antiplatelet drug was given concomitantly to 50.5% of patients. TE was diagnosed on 9 of 170 patients within a mean follow-up period of 2.44 years and the percentage per patient year of TE incidence was 2.17. Two patients had minor bleedings. On average, 29.3 TT (and PT-INR) measurements were made in 96 TE-free patients among 101 patients whose TT and PT-INR could be measured. The average TT value in TE-free patients was 21.1% (INR=1.73). On average, 11.6 TT measurements were made in 5 patients with TE and their mean TT was significantly higher at 26.4% (1.53) than in TE-free patients (p<0.01). There were no statistically significant differences in the frequency of TE between patients on warfarin alone (mean TT, 22.2%) and warfarin plus antiplatelet drugs (mean TT, 20.9%). This result might suggest that less TE complications occurred in our series despite less intensity of an oral anticoagulation terapy in Japan than Western countries.TT has been employed in only a few countries in the world. Although PT has been used in many countries, it's not suitable for international comparison due to different reagent sensitivity. Therefore, PT-INR can become an useful tool for monitoring the oral anticoagulation therapy, especially in the field of international studies.