SummaryAs recommcndcd by the proper authorities, the prothrombin time results for patients under oral anticoagulant therapy arc reported by our laboratory in International Normalized Ratios (I.N.R.).For the calculation of the INR-values we use a calibration factor, the International Sensitivity Index (I.S.I.), provided by the manufacturer of the thromboplastin reagent in use.Since standard reference plasmas with known INR-values are not yet available an«in-house»ISI calibration appears to be a useful internal quality control procedure.The imprccisions inherent to the use of INRvalues are almost negligible in clinical practice.Certainly they do not counter balance the overall advantage that all laboratories despite different thromboplastin reagents, can use identical therapeutic ranges for oral anticoagulation intensity.