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Adherence to INR monitoring in the community among VKA-treated patients in Saskatchewan : an observational study



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Background: Vitamin-K antagonists (VKA) are a class of oral anticoagulant medications used to prevent blood clots. The anticoagulant intensity of VKAs is measured with a blood test known as the International Normalized Ratio (INR). Traditionally, international guidelines have recommended INR tests every 4 weeks for all patients. However, adherence to these guidelines has never been investigated in real world settings. The objectives of this study were to describe adherence to INR testing in Saskatchewan among patients receiving VKA medications, and to identify predictors of optimal adherence. Methods: This was a retrospective cohort study of VKA users in Saskatchewan captured in the administrative data between 2003 and 2010. Physician claims for anticoagulation monitoring were used as a proxy for INR testing. Adherence to INR testing was measured using the Continuous, Multiple-Interval Measure of Medication Gaps (CMG). Individuals were considered adherent if adherence by the CMG was at least 80%. Hierarchical (random effects) logistic regression models were developed to identify important predictors of optimal INR monitoring. Individual physician identification was considered a random effect in these models. The dependent variable was the achievement of optimal adherence, defined as ≥80% adherence to the 4-week test interval. Results: Among 17,388 VKA users, 42% resided in rural areas and virtually all (99%) were monitored by a general practitioner. During a median follow-up of 514 days, 50% of patients exhibited at least 74% adherence to INR testing if a 4-week interval was used as the reference standard. However, the estimated median adherence increased dramatically to 98% when the benchmark for optimal testing was lengthened to every 12 weeks. The most prominent risk factors for poor adherence to INR monitoring appeared to be rural residence (rural vs. urban OR 0.55, 95% CI 0.47-0.64 among subjects age ≥75 years) and duration of VKA therapy (≥731 vs. 35-90 days OR 0.04, 95% CI 0.03-0.05). Discussion: Adherence to INR testing appeared to be acceptable for most VKA-treated patients in Saskatchewan. However, this data indicated that adherence might be more problematic in the subgroup of rural residents. Possible explanations include reduced access to testing facilities or the shortage of physicians in rural areas. Further research is required to understand if poor access is the underlying cause of non-adherence to INR testing in the rural population.



INR, international normalized ratio, adherence, compliance, VKA, vitamin-K antagonist, warfarin, anticoagulant



Master of Science (M.Sc.)


Pharmacy and Nutrition




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