Monitor warfarin efficacy by INR, experts say
Monitor warfarin efficacy by INR, experts say
Standard for stroke prevention seen as superior
Organizations that promote guidelines for stroke prevention endorse international normalized ratios (INR) as the standard for measuring warfarin therapy. The organizations American Heart Association, National Stroke Association, American Geriatrics Society, and American Society of Internal Medicine report recent findings that prothrombin time (PT), the most commonly used test to monitor oral anticoagulants, is no longer a safe or adequate measurement. A major limitation of the PT largely overcome by the use of the INR is the considerable variability in the results depending on the laboratory technique used.
The INR system standardizes the patient’s PT ratio using the International Sensitivity Index (ISI) of the particular thromboplastin. The ISI compares the locally used thromboplastin to an international standard, so the INR is the PT ratio that would have been obtained if the international standard had been used.
Advantages of the INR system include:
• uniform therapeutic ranges regardless of laboratory or country;
• effective comparison of clinical trial results;
• ease of monitoring.
It does have limitations. The INR system was intended for use in patients who have been on oral anticoagulant therapy for several weeks. Because of the varying sensitivities of individual thromboplastins, there may be variability in INRs generated by different laboratories. The INR may also be less useful when screening for coagulation problems such as those associated with liver disease. Using thromboplastins with low ISI values may help to minimize the potential problems associated with INR.
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