International Normalized Ratio (INR), which standardizes prothrombin time (PT) during oral anticoagulation, has been extended to standardize PT in liver diseases and is included in prognostic models such as the Model for End stage Liver Disease (MELD). However, mechanisms of PT prolongation in liver diseases differ from those involved in oral anticoagulation, and the thromboplastin reagents differ in their sensitivities to these 2 mechanisms. Our aim was to determine whether, in the calibration model for thromboplastins proposed by the World Health Organization, the use of plasmas from patients with liver diseases instead of plasmas from patients on oral anticoagulation could lead to a new INR specific for liver diseases (INR "LD"), achieving a real standardization of PT. First, 5 thromboplastins were calibrated against an international reference using 60 plasmas of patients with liver failure and, in a second step, the variation of PT reported as seconds, the ratio of patient PT to normal PT, See Editorial on Page 295 P rothrombin time (PT) is the usual test for monitoring oral anticoagulation by vitamin K antagonists 1 and is widely adopted to assess the severity of acute and chronic liver injury. 2 PT is the recalcification time of citrated plasma in the presence of a thromboplastin reagent. 3 The term thromboplastin refers to a complex mixture of tissue factor and phospholipids prepared from tissue extracts of animal or human origin. Recent thromboplastins contain recombinant tissue factor. PT is influenced by the levels of the following coagulation factors: I (fibrinogen), II, V, VII, and X, all factors being synthesized by the liver and, among them, 3 (II, VII, and X) being vitamin K dependent. Thromboplastins can vary markedly in their responsiveness to the defects induced by vitamin K antagonist therapy. As a result, the poor agreement between PT in different laboratories whatever the methods of reporting results 4 has led to important differences in oral anticoagulant dosing. 5,6 Standardization of PT reporting in patients on oral anticoagulation has been improved by the calibration model for thromboplastins proposed by the World Health Organization (WHO). 7 This system defines, for a specific thromboplastin reagent, the International Sensitivity Index (ISI) obtained by a calibration using plasmas of patients on stable oral anticoagulant doses, against a WHO international reference thromboplastin. 7 The ISI is a numerical value that reflects the responsiveness of a given thromboplastin to reduction of the vitamin K-dependent coagulation factors; the lower the ISI value, the greater the sensitivity of the reagent. The recommended scale of reporting PT results is