Harmonizing the International Normalized Ratio (INR): Standardization of methods and use of novel strategies to reduce interlaboratory variation and bias

OBJECTIVES: To reduce interlaboratory variation and bias in international normalized ratio (INR) results, as used to monitor patients receiving vitamin K antagonist therapy, including warfarin, in a large pathology network (n = 27 laboratories) by procedural standardization and harmonization.

METHODS: Network consensus to standardize to common instrument and reagent platforms was established, following development of hemostasis test specifications. Subsequent installations and implementation occurred after conclusion of a government tender process. Network-wide application of simple novel process of verification harmonization of local international sensitive index and mean normal prothrombin time initiated for each new lot of INR reagent that does not require ongoing use of reference thromboplastin or calibration/certified plasma sets.

RESULTS: We achieved reduction of different instrument manufacturers (from four to one), instrument types (10 to three), reagent types (four to one), and instrument/reagent combinations (12 to three), plus substantial reduction in INR variability and bias.

CONCLUSIONS: Results infer significant improvement in local patient management, with positive implications for other laboratories. For the United States in particular, lack of US Food and Drug Administration-cleared certified plasmas may compromise INR accuracy, and our novel approach may provide a workable alternative for laboratories and networks.