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Can People Who Are Receiving Steady Doses of Warfarin Have Their Dose Assessed Less Frequently? FREE

[+] Article and Author Information

The full report is titled “Warfarin Dose Assessment Every 4 Weeks Versus Every 12 Weeks in Patients With Stable International Normalized Ratios. A Randomized Trial.” It is in the 15 November 2011 issue of Annals of Internal Medicine (volume 155, pages 653-659). The authors are S. Schulman, S. Parpia, C. Stewart, L. Rudd-Scott, J.A. Julian, and M. Levine.


Ann Intern Med. 2011;155(10):I-34. doi:10.7326/0003-4819-155-10-201111150-00001
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What is the problem and what is known about it so far?

Warfarin is a medicine that “thins” the blood. Doctors prescribe it to treat or prevent blood clots. Use of the drug requires testing to see if the blood is thinned too little, too much, or just enough. Blood that is too thin requires withholding or decreasing a warfarin dose. Blood that is not thin enough requires increasing a dose. Once testing shows that a person is receiving the proper dose for the right amount of blood thinness, the same dose can be used in some people indefinitely. Other people require frequent dose adjustments. In theory, people whose blood thinness does not change with a steady dose of warfarin can safely go longer periods between times when their warfarin dose is assessed by their doctors and changed if needed.

Why did the researchers do this particular study?

To test the safety of extending the period between warfarin dose assessment for people receiving steady doses.

Who was studied?

250 people receiving warfarin whose dose was unchanged for at least 6 months.

How was the study done?

The researchers tested participants' blood thinness every 4 weeks for 1 year. They randomly assigned the participants to 2 groups. In the first group, the true results of every blood test were reported to doctors (4-week group). In the second group, the researchers changed the results of 2 out of every 3 tests to make it look like the person was receiving the proper amount of warfarin. In that way, the participants' doctors had the information required to change the dose if it needed to be changed only once every 12 weeks (12-week group). The researchers then calculated the amount of time that participants' blood was measured as being thin enough and assessed how often participants had complications of warfarin treatment, such as bleeding or clots.

What did the researchers find?

The amount of time that participants' blood was thin enough was the same in the 2 groups. Participants in the every-12-week group had fewer warfarin dose adjustments. The researchers detected no other differences between groups in complications of treatment.

What were the limitations of the study?

The research took place at a single medical center and warfarin clinic. The findings might be different in settings with different doctors and testing processes. Participants in the every-12-week group still talked to warfarin clinic staff every 4 weeks and received information about maintaining a steady level of blood thinness and warfarin dose. The findings might be different if such contact occurs only every 12 weeks when their blood is tested that infrequently.

What are the implications of the study?

For people receiving a steady warfarin dose for at least 6 months, dosing can probably be assessed and changed if needed once every 12 weeks, provided that people also communicate with warfarin clinic staff every month. All others should continue to have testing and dose assessments made every 4 weeks or at a frequency recommended by their doctors.

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