Characteristics associated with consistently therapeutic International Normalized Ratios

Objectives: To establish patient characteristics associated with an increased likelihood of an International Normalized Ratio (INR) in therapeutic range.

Methods: A total of 200 patients who had anticoagulation therapy monitored at the Charles George VA Medical Center (CGVAMC) for 6 months with an INR range of either 2-3 or 2.5-3.5 were retrospectively reviewed.  Characteristics evaluated for influence on INR stability included age >70, gender, primary indication for anticoagulation therapy, INR target range, diabetes mellitus, hypertension, heart failure, prior venous thrombosis, concomitant use of antibiotics, and number of concomitant medications.

Results: A statistically significant difference was found showing greater INR stability in patients without diabetes in comparison to patients with diabetes (59 +/- 16 versus 54 +/- 17, p = 0.0183).  A multivariate regression analysis demonstrated concomitant use of antibiotics had a negative impact on INR stability.  In patients > 70 years old, a trend toward INR stability was shown in comparison to patients < 70 years of age (59 +/- 16 versus 54 +/- 17, p = 0.0532).  No association with heart failure and INR stability was found. 

Conclusion: Absence of diabetes can be used to identify patients eligible for extended monitoring intervals as this patient population was shown to have greater INR stability measured by number of visits with INR in therapeutic range.  Recent evaluations suggest warfarin can remain the most cost effective anticoagulation regimen despite newer therapies that do not require monitoring.  This study contributes to the basis for evaluating optimization of warfarin monitoring intervals which allows health systems to define cost effective alternatives to newer therapies.    

This material is the result of work supported with resources and the use of facilities at the Charles George VA Medical Center, Asheville, NC.