Effect of patient-specific factors on weekly warfarin dose

Heather P Whitley¹, Joli D Fermo², Elinor CG Chumney^2,3, Walter Adam Brzezinski³

¹Auburn University Harrison School of Pharmacy, Department of Pharmacy Practice and The University of Alabama School of Medicine, Tuscaloosa, Department of Community and Rural Medicine, Tuscaloosa, AL USA; ²South Carolina College of Pharmacy, Department of Pharmacy and Clinical Sciences, Charleston, SC USA; ³Medical University of South Carolina, College of Medicine, Charleston, SC USA

Abstract

Objective: To determine the influence of various patient-specific factors, use of concomitant medications, and weekly vitamin K intake on total weekly warfarin maintenance dose (TWD).

Methods: Information collected, via retrospective chart review, included TWD, general demographics, vitamin K consumption, target INR range, use of alcohol, tobacco, and cytochrome P450 (CYP)-inducing medications, and concomitant medications and diseases.

Results: The majority of patients (n = 131) were Caucasian (71%), with more females (55%) than males. Use of CYP-inducing medications resulted in the largest correlation coefficient (r = 0.30). The sample was divided into high warfarin dose (TWD ≥ 50 mg) and low warfarin dose (TWD ≤ 25 mg) patient populations to discern areas where the two populations differed. Age and amiodarone use were the only statistically significant differences between the two groups, with high dose patients tending to be younger and to use less amiodarone. Age and CYP-inducing medications were found to be the only statistically significant predictors of TWD in the regression analysis. The TWD was 2.4 mg less for each additional decade of patient age. The coefficient on CYP-inducing medications indicates that the concomitant use of a CYP inducer is associated with an increase in TWD of 17.2 additional milligrams, adjusting for all other variables in the model.

Conclusion: We found concomitant use of CYP inducer, age, height, and ethnicity to have the greatest influence on TWD. Positive relationships were found between TWD and the use of CYP450 inducer, height, and African American ethnicity. Although it did appear that women required a lower TWD than men, this factor contributed mildly. Further studies with a greater sample size may more precisely predict the effect of patient-specific factors on TWD, thus uncovering additional relationships.

Keywords: anticoagulation, survey, vitamin K, warfarin, CYP450 inducers