International normalized ratio.

Dear Madam, The P.T. assay, first described in 1935 by Quick et al. soon became the monitoring of oral anticoagulant therapy. The P.T. measures the affect of reduction in the Vit. K dependent coagulant factors II, VII and X. Because the Prothrombin Time Ratio (P.T.R.) is markedly affected by the activity of thromboplastin, the W. It helps to determine the intensity of anti-coagulation therapy based on results of prothrombin time tests performed by laboratories using different normal ranges. In many coutries, laboratories report I.N.R. as if the prothrombin time (P.T.) had been determined with WHO. Reference thromboplastin (I.S.I). Many manufacturers now calibrate their thromboplastin reagent against W.H.O. reference standard and provide the user with an I.S.I. value enabling hospital laboratories to report result as I.N.R. in standardized manner instead of P.T.R. The difference in responsiveness of thromboplastin is a major potential cause of variability of " Prothrombin Time " test-results. Because of the variability in the commercial thromboplastin used to determine P.T. and consequent uncertainty about the actual intensity of anti-coagulation. may reduce the potential gain in life expectancy.