|
||
|
American Journal of Medicine
|
|
|
||
|
American Journal of Medicine
|
Another challenge with warfarin is the necessity to carefully control the dose because of several patient variables, including individual genetic differences, comorbidities, and comedications that may interact with the drug. Food and alcohol also have interactions with warfarin.
Serum levels of warfarin should be maintained at an international normalized ratio (INR) between 2.0 and 3.0 – and in some countries this therapeutic range is well achieved, whereas in others it is not. For example, data from the study by Baker and colleagues (lefthand side of Figure 7)[10] show that the average overall time in the therapeutic range (TTR) was only 51%. The data on the right of the Figure[11] show the consequence of this low TTR: for optimal effectiveness warfarin needs to be in the therapeutic range at least 70% of the time and when it is, the likelihood of stroke or systemic embolism is very small. As the patients spend less and less time in the therapeutic range, the likelihood of these adverse events becomes greater and greater. In fact, if the TTR is <40% or <50%, the result is actually worse than not using any warfarin at all. Finally, If this kind of analysis is restricted to patients who have a relatively high risk of stroke – say, a CHADS2 score ≥2 – then the TTR has to be >70% in order to secure clinical benefit. Camm J. Am J Med 2013; published on-line at http://education.amjmed.com/00000.
[10] Baker WL, Coleman CI, Kluger J, et al. Systematic review: comparative effectiveness of angiotensin-converting enzyme inhibitors or angiotensin II-receptor blockers for ischemic heart disease. Ann Intern Med. 2009;151:861-871.
[11] Morgan CL, McEwan P, Tukiendorf A, et al. Warfarin treatment in patients with atrial fibrillation: observing outcomes associated with varying levels of INR control. Thromb Res. 2009;124:37-41.