The International Normalized Ratio (INR) for Monitoring Warfarin Therapy: Reliability and Relation to Other Monitoring Methods

  1. Dzung The Le, MD, PhD;
  2. Robert T. Weibert, PharmD;
  3. Barbara K. Sevilla, MT;
  4. Kathleen J. Donnelly, MS; and
  5. Samuel I. Rapaport, MD
  1. From the University of California, San Diego, La Jolla, California. Requests for Reprints: Samuel I. Rapaport, MD, UCSD Medical Center, Mail Code 8423, 200 West Arbor Drive, San Diego, CA 92103. Grant Support: By grant HL 27234 from the National Heart, Lung, and Blood Institute, National Institutes of Health.

    Abstract

    Objective: To enhance understanding of the reliability of the international normalized ratio (INR) for monitoring warfarin therapy and its relation to other monitoring techniques.

    Design: Prospective cohort study.

    Setting: A university hospital.

    Patients: 79 patients attending an anticoagulation clinic.

    Measurements: International normalized ratios obtained with a portable capillary monitor (Coumatrak) and the following from a simultaneous plasma sample: INRs from prothrombin times done with six thromboplastins, prothrombin-proconvertin (P&P) test activity, specific prothrombin activity, and native prothrombin antigen.

    Results: Converting to INRs failed to standardize prothrombin time results obtained with high- and low-sensitivity thromboplastins. Coumatrak INRs correlated best with INRs obtained with high-sensitivity thromboplastins. The INR range of 2.0 to 3.0 corresponded to a P&P range of 30% to 13%, a native plasma prothrombin antigen range of 56 to 24 µg/mL, and a specific prothrombin activity range of 43% to 21%.

    Conclusions: Low-sensitivity thromboplastins may give erroneously high INRs in the upper therapeutic range. Plasma prothrombin times should be done with a high-sensitivity thromboplastin, particularly in patients maintained at the upper limit of the therapeutic range. An INR so obtained correlated well with an INR obtained with a portable capillary blood monitor.

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    1. Ann Intern Med April 1, 1994 vol. 120 no. 7 552-558
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