International Normalized Ratio Monitoring of Vitamin K Antagonist Therapy: Comparative Performance of Point-of-Care and Laboratory-Derived Testing

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Abstract

The monitoring of warfarin therapy using the international normalized ratio (INR) has now moved outside the laboratory's control by use of point-of-care (POC) devices. Although this provides patients with the convenience of immediate results and clinical assessment, POC-INRs are often performed by nonlaboratory staff with little experience in quality control. The Royal College of Pathologists of Australasia Quality Assurance Program (RCPAQAP) Haematology has devised a POC-INR external quality assessment (EQA) program that is suitable for both laboratory and nonlaboratory operators (e.g., nurses) to perform INR testing with good accuracy and precision. A comparison of the performance of the POC versus the laboratory-derived INR testing over the past 8 years has shown that the variation in test results (expressed as coefficient of variation; CV) for laboratory INRs increases with more prolonged INR values, whereas CVs for the POC-INR testing were generally lower, with a reduced dependency on INR values. In our program, the CoaguChek XS (Roche, Basel, Switzerland) showed the best performance among the POC devices. A comparative assessment with other EQA providers showed agreement and disparity with our data in terms of comparative CVs obtained between the laboratory and POC-INRs. The growth of the RCPAQAP POC-INR program from 29 to 360 in the past 12 years highlights the importance of providing suitable EQA for POC-INR staff who are unfamiliar with laboratory practice. This helps maintaining consistent results, which have important implications for the therapeutic management of patients on vitamin K antagonist therapy.

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