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Letter to the Editor |
Department of Medicine (Endocrinology), Mubarak Al-Kabeer Teaching Hospital and Kuwait University, Kuwait, sardoi{at}gmx.net
Received: June 25, 2007.
Revised: October 18, 2007.
Accepted: October 25, 2007.
Key Words: Warfarin • Anticoagulation • Nomogram • International normalization ratio (INR)
Editor - Warfarin is a potential life saving agent but the risks of warfarin therapy are nevertheless substantial so appropriate dosage titration is mandatory. The individualization of warfarin therapy has traditionally been empiric but because significant numbers of patients are either under-treated or over-treated,1,2 methods have been devised to predict optimal warfarin doses. This is because it has been noted that the eventual dose required varies widely between apparently similar people, and the consequences of either under-anticoagulation or over-anticoagulation are clinically important.3 Also, the time taken to initiate treatment and decide the correct maintenance dose by the traditional empirical method often takes several weeks.
Many models for the initiation and control of warfarin therapy have been proposed and I read with interest the dose prediction method of Caldwell et al4 published in the last issue of Clinical Medicine & Research which explained 56% of the variation in the maintenance dose. The dosing algorithm made use of genetic testing that is complex, only feasible in the United States, and not currently available to clinical practice.4 This reminded me of the pharmacodynamic model5,6 I had proposed in 1994 and later modified in 2001 with results that have been validated in two different cohorts6 and translated into a simple nomogram for clinical use (figure 1
). This method had then explained 73% of the variation in the maintenance dose and required no special test, and was very easy to implement and use. The fact that Caldwell et al were contemplating such a difficult system, given the availability of simpler yet similarly predictive systems, prompted me to write about this method.
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2. Follow-up is usually in the hospital’s anticoagulation clinic run by the hematology service or medical outpatients. My experience with the use of this model for dose prediction in Kuwait has been excellent, and indeed in a random selection of 13 patients this model explained 97% of the interindividual variation in warfarin dose requirements.
With a simple predictive model such as this that explains 73% or more of the interindividual variability in warfarin maintenance dose requirements, I think it would be safe to conclude that the use of this model in the form of the included nomogram (figure 1
) should make it less appealing to come up with more complex methods, such as that suggested by Caldwell et al4 of decreasing both the occurrence of either under-anticoagulation or over-anticoagulation, as well as the time taken to initiate treatment and decide the correct maintenance dose during the initiation of oral anticoagulation with warfarin in hospitals. It is also clinically feasible and very simple to implement. I would, therefore, like to bring this to the attention of physicians involved with anticoagulation who are contemplating other methods for the initiation of warfarin in hospitals.
References
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