Dzung The Le, MD, PhD; Robert T. Weibert, PharmD; Barbara K. Sevilla, MT; Kathleen J. Donnelly, MS; Samuel I. Rapaport, MD
To enhance understanding of the reliability of the international normalized ratio (INR) for monitoring warfarin therapy and its relation to other monitoring techniques.
Prospective cohort study.
A university hospital.
79 patients attending an anticoagulation clinic.
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.
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%.
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.
Le DT, Weibert RT, Sevilla BK, Donnelly KJ, Rapaport SI. The International Normalized Ratio (INR) for Monitoring Warfarin Therapy: Reliability and Relation to Other Monitoring Methods. Ann Intern Med. ;120:552–558. doi: 10.7326/0003-4819-120-7-199404010-00004
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Published: Ann Intern Med. 1994;120(7):552-558.
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