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Related Experiment Videos

Standardizing the prothrombin time. Calibrating coagulation instruments as well as thromboplastin

D M Becker1, J E Humphries, F B Walker

  • 1Department of Medicine, University of Virginia Health Sciences Center, Charlottesville 22908.

Archives of Pathology & Laboratory Medicine
|June 1, 1993
PubMed
Summary

Differences in prothrombin time (PT) monitoring instruments can significantly impact warfarin dosing, even with International Sensitivity Index (ISI) adjustments. Standardizing coagulation instruments is crucial for safe and effective warfarin therapy.

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Area of Science:

  • Clinical Pathology
  • Pharmacology
  • Medical Instrumentation

Background:

  • Warfarin anticoagulation indications are rising, necessitating reliable monitoring.
  • Point-of-care and home prothrombin time (PT) monitoring are increasingly common.
  • International Sensitivity Index (ISI) aims to standardize warfarin dosing but may not fully account for instrument variability.

Purpose of the Study:

  • To systematically compare prothrombin time (PT) and international normalized ratio (INR) results between a portable clinic monitor and a hospital reference laboratory.
  • To assess the clinical significance of discrepancies in PT/INR measurements from different coagulation instruments.

Main Methods:

  • Two groups of 50 patients on stable warfarin doses were studied over 3 months.
  • Prothrombin times (PTs) and international normalized ratios (INRs) were measured independently from single venipuncture samples.

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  • Regression analysis was used to compare results from the portable monitor and the hospital reference laboratory.

    • Main Results:

    • While PT/INR results showed high correlation between instruments (r values ranging from 0.88 to 0.94), they were not identical.
    • Significant discrepancies were observed, with only 82% of all sample differences falling within 1.0 INR unit.
    • Regression analysis revealed systematic differences between the monitor and reference laboratory measurements.

    Conclusions:

    • Coagulation instrument variability can lead to clinically meaningful differences in warfarin therapy.
    • Calibration of both coagulation instruments and thromboplastins is essential for standardizing warfarin dosing and ensuring patient safety.