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

Dzung The Le, MD, PhD; Robert T. Weibert, PharmD; Barbara K. Sevilla, MT; Kathleen J. Donnelly, MS; and Samuel I. Rapaport, MD
[+] Article and Author Information

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.


Copyright ©2004 by the American College of Physicians


Ann Intern Med. 1994;120(7):552-558. doi:10.7326/0003-4819-120-7-199404010-00004
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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.

Figures

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Figure 1.
Plots of Coumatrak monitor data as a function of plasma prothrombin time data.A.rnB.INRsrn

plot of Coumatrak monitor prothrombin time ratios (PTRs) as a function of plasma prothrombin time ratios obtained with a high sensitivity thromboplastin (Dade IS). The Equation for the linear regression line is Y = 0.36X + 0.88, = 0.87, = 100. A plot of Coumatrak monitor international normalized ratios ( ) as a function of plasma prothrombin time INRs obtained with Dade IS thromboplastin. The Equation for the linear regression line is Y = 0.84X + 0.62, = 0.90, = 100.

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Figure 2.
Linear regression lines derived from prothrombin time ratios and from international normalized ratios.A.B.[6]

The linear regression lines obtained by plotting prothrombin time ratios (PTRs) determined with five thromboplastins and with the Coumatrak monitor as a function of prothrombin time ratios obtained with Dade IS. The linear regression lines obtained after converting the data of panel A to international normalized ratios [INRs]. In both panels A and B, the thromboplastins are numbered as follows: 1) Ortho; ISI, 2.30; 2) Dade C; ISI, 2.88; 3) Dade C-plus; ISI, 2.03; 4) Excel-S; ISI, 1.34; 5) Excel; ISI, 1.99 or 2.04 for two lots used. Line 6 is the regression line for the Coumatrak monitor data .

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Figure 3.
Plots of plasma prothrombin times expressed as 1/INR as a function of percent prothrombin-proconvertin (P&P) activity.A.B.

A primary plot of 1/INR values obtained with Dade IS as a function of values for percent P&P activity. The linear regression lines of 1/INR values obtained from plasma prothrombin times done with each of the six thromboplastins plotted as a function of percent P&P activity. INR = international normalized ratio.

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Figure 4.
Plots of plasma prothrombin times expressed as 1/INR as a function of plasma native prothrombin antigen (NPA) levels.A.B.

A primary plot of 1/INR values obtained with Dade IS as a function of values for native prothrombin antigen. The linear regression lines of 1/INR values obtained from plasma prothrombin times done with each of the six thromboplastins plotted as a function of native prothrombin antigen. INR = international normalized ratio.

Grahic Jump Location
Grahic Jump Location
Figure 5.
Plots of plasma prothrombin times expressed as 1/INR as a function of percent specific plasma prothrombin activity.A.B.

A primary plot of 1/INR values obtained with Dade IS as a function of values for specific plasma prothrombin activity. The linear regression lines of 1/INR values obtained from plasma prothrombin times done with each of the six thromboplastins plotted as a function of specific plasma prothrombin activity. INR = international normalized ratio.

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