Defining a metrologically traceable and sustainable calibration hierarchy of International Normalized Ratio (INR) for monitoring of vitamin K antagonist treatment in accordance with International Standard ISO 17511:2020: Communication from the IFCC-SSC/ISTH working group on PT/INR standardization.

Calibration of Prothrombin Time (PT) in terms of the International Normalized Ratio (INR) has been outlined in 'Guidelines for thromboplastins and plasmas used to control oral anticoagulant therapy' (World Health Organization, 2013). International Standard ISO 17511:2020 presents requirements for manufacturers of In-vitro Diagnostic Medical Devices (IVD MD) in documenting the calibration hierarchy for a measured quantity in human samples using a specified IVD MD. The objective of this article is to define an unequivocal, metrologically traceable calibration hierarchy for the INR measured in plasma as well as in whole blood samples. Calibration of PT and INR for IVD MD according to WHO Guidelines is similar to cases where there is a reference measurement procedure that defines the measurand for value assignment as described in ISO 17511:2020. We conclude that for PT/INR standardization the optimal calibration hierarchy includes a primary process to prepare an international reference reagent and measurement procedure that defines the measurand by a value assignment protocol conforming to clause 5.3 in ISO 17511:2020. A panel of freshly prepared human plasma samples from healthy adult individuals and patients on vitamin K antagonists is used as a commutable secondary calibrator as described in ISO 17511:2020. A sustainable metrologically traceable calibration hierarchy for INR should be based on an international protocol for value assignment with a single primary reference thromboplastin and the harmonized manual tilt tube technique for clotting time determination. The primary international reference thromboplastin reagent should be used only for calibration of successive batches of secondary reference thromboplastin reagent.