INTRODUCTION: Warfarin is an anticoagulant that is difficult to administer 
because of its narrow therapeutic margin and the numerous factors that influence 
patient response.
OBJECTIVE: Demographic, clinical and genetic variables were characterized to 
establish the appropriate maintenance dosages of warfarin.
MATERIALS AND METHODS: The Colombian patients consisted of 145 adults of both 
sexes. They were in stable anticoagulation status with international normalized 
ratio between 2 and 3 for at least two months, and without changes in the 
warfarin commercial preparation or in the dosage. After signing the informed 
consent, the following data was recorded for each volunteer: age, gender, 
weight, height, smoker status, co-morbidity, co-medication, International 
Normalized Ratio (INR), warfarin dose, and commercial brand. Each patient was 
typed for genes CYP2C9, VKORC1, CYP4F2 and PROC; for 59 patients, the serum 
levels of warfarin were quantified. The genotyping and the blood quantification 
were performed by mini-sequencing and HPLC methods, respectively.
RESULTS: Age, co-medication with enzymatic inhibitors (amiodarone, sertraline, 
fluoxetine) or inducers (phenytoin, carbamazepine), and the alleles rs1799853 
(*2) and rs1057910 (*3) of the CYP2C9 gene, as well as rs9923231 of the VKORC1 
gene were associated with warfarin dose required to achieve anticoagulation with 
INR of 2-3. These variables were included in a multiple linear regression model 
for predicting the optimum dose/week of warfarin. This resulted in an algorithm 
that explained 47.4% of the variability in the dose responses.
CONCLUSION: Clinical and pharmacogenetic variables provided a basis for 
improving the safety and effective dosage of warfarin; however, the use of a 
pharmacogenetic algorithm will require patient data obtained during clinical 
trials.