Environmental and individual predictors of medication adherence among elderly patients with hypertension and chronic kidney disease: A geospatial approach
Introduction
Angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin-receptor blockers (ARBs) are recommended by practice guidelines as preferred anti-hypertensive agents for Chronic Kidney Disease (CKD) patients because of their additional protective renal benefits.1,2 Adherence to anti-hypertensive treatment is crucial for patients with hypertensive CKD, as poor medication adherence may result in uncontrolled blood pressure, and further, accelerate the rate of CKD progression and increase the risk of hospitalization, cardiovascular conditions, and death.3, 4, 5, 6 Previous research using nationally representative data has shown that approximately only one-third of CKD patients in the United States had their blood pressure under control.7 Despite the importance of anti-hypertensive regimens, adherence to these agents remains suboptimal in this population. Previous studies of medication adherence have found that approximately 65% - 83% of CKD patients were adherent to their prescribed anti-hypertensive agents, while studies using self-report measures demonstrated somewhat better adherence rates than those using prescription refill measures (67%-83% versus 65%-70%).3, 4, 5,8,9
Reasons for poor adherence to anti-hypertensive treatments in CKD patients vary from study to study and have been attributed to distinct characteristics of investigated medications and populations. For example, individuals' social and demographic factors such as younger age, male sex, lower level of income and education were associated with increased risks of poor adherence in some studies but not in others.3,4,8,10 With regards to patient health status factors, being depressed, having more hospitalizations, and unable to self-administer medications have been associated with poor adherence.4,5,8 Inconsistent relationships between medication adherence and renal function have been observed in previous research.5,10,11 Interview-based and survey-based studies have found that forgetfulness was the most common reason for nonadherence reported by CKD patients.3,4,12 Adherence with anti-hypertensive treatments in CKD patients has shown to be influenced by other subjective factors, such as, patients’ perceived need for medication, perceived efficacy of medication, concerns about side effects, as well as physician-patient communication.12,13 When treatment related characteristics were examined, medication side effects, complexity of regimens, and overall pill burden were associated with poor medication adherence.8,14
Although many studies have explored predictors of poor cardiovascular medication adherence, very few have examined how medication adherence varies across different regions or how neighborhood-level factors may be related to individuals’ medication-taking behaviors. A recently published study by Erickson et al. found geographical clustering in adherence to statins in the state of Michigan in the United States.15 Similarly, another study by Hoang et al. observed spatial clustering in medication adherence among 1081 patients residing in southeastern Michigan who were discharged with acute coronary syndrome conditions.16 A study by Couto et al. found that across the United States, adherence rates were highest in New England and the West North Central region, and followed by the East North Central and the Middle Atlantic region,17 while the entire southern section of the United States, including the West South Central, the East South Central, and the South Atlantic region had relatively poor adherence. Moreover, similar geographical variation was observed in both Medicare beneficiaries and commercial insurance beneficiaries, and the variation was stable across different therapeutic drug classes (antidiabetics, antihypertensives, and antilipidemics). However, these studies did not investigate local characteristics that might contribute towards the geographic differences in medication adherence.
According to the Behavioral model of health services use proposed by Andersen, patients' utilization of health care is influenced by not only characteristics of patients, but also environmental factors, such as structures of the health system and neighborhood socioeconomics.18,19 Identifying environmental risk factors of medication nonadherence could be helpful in designing population-based strategies to impact health promotion. Therefore, the aim of this study was to explore local variations in medication adherence of ACEIs/ARBs, a commonly used class of recommended antihypertensive medications and examine environmental and individual influences on medication adherence. We hypothesized that medication-taking behaviors, defined in this study as adherence to prescribed ACEIs/ARBs, are associated with both patients’ individual characteristics and the characteristics of the neighborhood they live in. Moreover, we expected that the adherence rate of ACEIs/ARBs would vary across regions in the United States. It was also expected that the relationship between environmental factors and medication adherence would vary across the United States. In this study, a Geographically Weighed Regression (GWR) model was used to test the working hypotheses.
Section snippets
Data source
This study used the Medicare 5% sample files from the United States Renal Data System (USRDS) database. These files contain comprehensive information on demographic characteristics, Medicare enrollment status, diagnoses, procedures, and filled prescriptions for a random 5% sample of Medicare beneficiaries across the United States over time. To assess the relationship between environmental factors and medication-taking behaviors, Medicare claims were further merged with external data resources
Medication-related outcome measures
One-year ACEIs/ARBs adherence was measured by PDC, which was defined as the proportion of days covered by ACEIs/ARBs in a fixed one-year refill interval, starting from the first date of dispensing ACEIs/ARBs. In this study, ACEIs and ARBs were treated as the same. A threshold of 80% PDC was applied to define good medication adherence, with higher PDC associated with greater adherence.22
Individual factors
Demographic information such as age, gender and race were extracted from the Medicare 5% files. Age was
Statistical analyses
Study subjects were categorized into two groups based on their medication adherence: a PDC of at least 80% vs. a PDC below 80%. Patients with PDC above 80% were considered as being adherent to prescribed ACEIs/ARBs. Descriptive analyses of individual characteristics and environmental factors were conducted at baseline. The means and standard deviation for continuous variables and percentages for categorical variables are presented in Table 1. Group differences in these factors were examined
Results
The study cohort consisted of 70,201 aged Medicare beneficiaries with hypertension and CKD who used ACEIs/ARBs during the study period. These patients resided in a total of 2,981 counties in the United States, with an average of 24 persons per county. Approximately 61% of them had one-year PDC of at least 80% (mean = 0.9, SD = 0.1) while the rest of them had poor adherence (mean = 0.5, SD = 0.2). Table 1 reports the descriptive statistics of individual and environmental characteristics between
Discussion
This study is, to the best of the authors' knowledge, the first nationally representative study to examine the spatial variation in adherence to ACEIs/ARBs among CKD patients and investigate the spatial association between environmental factors and medication adherence across the United States. Approximately 61% of study subjects were adherent to their prescribed ACEIs/ARBs, defined as having a PDC at least 80%, which is lower than the adherence rate demonstrated in previous studies using
Conclusion
Medication adherence to ACEIs/ARBs varied across the United States, with the Northeast and Midwest showing better adherence than the South. Different risk factors at both the individual level and the environmental level have been detected in this study. Additionally, the study displays the geographic variation of the relationship between environmental characteristics and medication adherence, which helps to better clarify the place effects behind these relationships. This information has
Funding/disclosure
This research is funded through the United States Renal Data System (USRDS). The USRDS is funded by NIDDK, through NIH contract HHSN276201400001C. The USRDS Coordinating Center is located at the Kidney Epidemiology and Cost Center, University of Michigan, in partnership with Arbor Research Collaborative for Health, Ann Arbor, Michigan. None of the authors have any conflicts of interest to declare.
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