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J Gen Intern Med. 2011 December; 26(12): 1479–1491.
Published online 2011 August 20. doi:  10.1007/s11606-011-1825-3
PMCID: PMC3235604

A Systematic Review of Adherence to Cardiovascular Medications in Resource-Limited Settings

Ashna D. K. Bowry, MBChB, CCFP,1,2 William H. Shrank, MD, MHSc,1 Joy L. Lee, MS,1 Margaret Stedman, PhD,1 and Niteesh K. Choudhry, MD, PhDcorresponding author1

Abstract

Background

Medications are a cornerstone of the prevention and management of cardiovascular disease. Long-term medication adherence has been the subject of increasing attention in the developed world but has received little attention in resource-limited settings, where the burden of disease is particularly high and growing rapidly. To evaluate prevalence and predictors of non-adherence to cardiovascular medications in this context, we systematically reviewed the peer-reviewed literature.

Methods

We performed an electronic search of Ovid Medline, Embase and International Pharmaceutical Abstracts from 1966 to August 2010 for studies that measured adherence to cardiovascular medications in the developing world. A DerSimonian-Laird random effects method was used to pool the adherence estimates across studies. Between-study heterogeneity was estimated with an I2 statistic and studies were stratified by disease group and the method by which adherence was assessed. Predictors of non-adherence were also examined.

Findings

Our search identified 2,353 abstracts, of which 76 studies met our inclusion criteria. Overall adherence was 57.5% (95% confidence interval [CI] 52.3% to 62.7%; I2 0.98) and was consistent across study subgroups. Studies that assessed adherence with pill counts reported higher levels of adherence (62.1%, 95% CI 49.7% to 73.8%; I2 0.83) than those using self-report (54.6%, 95% CI 47.7% to 61.5%; I2 0.93). Adherence did not vary by geographic region, urban vs. rural settings, or the complexity of a patient’s medication regimen. The most common predictors of poor adherence included poor knowledge, negative perceptions about medication, side effects and high medication costs.

Interpretation

Our study indicates that adherence to cardiovascular medication in resource-limited countries is sub-optimal and appears very similar to that observed in resource-rich countries. Efforts to improve adherence in resource-limited settings should be a priority given the burden of heart disease in this context, the central role of medications in their management, and the clinical and economic consequences of non-adherence.

KEY WORDS: cardiovascular medications, cardiovascular disease, compliance, cardiovascular risk reduction

Non-infectious chronic diseases have long been thought to primarily affect affluent populations. However, these conditions are responsible for more deaths, both in absolute numbers and relative proportions, in resource limited settings.1 Cardiovascular disease imposes a particular burden and is the leading cause of death in all age groups in virtually all low and middle income nations. Its prevalence in these regions is increasing at more than twice the rate observed in resource-rich countries.1 Thus, the prevention and management of cardiovascular illness has become a major focus of healthcare providers worldwide.1

Medications are a cornerstone of cardiovascular risk reduction.2 In resource-rich settings, substantial effort has been devoted to improving appropriate prescribing.2 However, longer-term adherence to evidence-based medications remains suboptimal.2 For example, only half of patients who experience an acute coronary event are adherent to their prescribed statin two years after starting therapy.3,4

Despite its disproportionate share of disease burden, much less is known about medication adherence in resource-limited regions. Access to healthcare, cultural beliefs, education about chronic disease and the role of medication, the nature of patient-physician interactions and social supports, among many other factors, are very different in resource-limited countries and may profoundly affect rates of adherence.5,6 A greater understanding of these factors will help in the development of quality improvement activities in this context. Accordingly, we systematically reviewed the published literature in order to evaluate prevalence and predictors of non-adherence to cardiovascular medications in resource-limited settings.

METHODS

We performed an electronic search of Ovid Medline, Embase and International Pharmaceutical Abstracts from January 1, 1966 to August 19, 2010 for studies that reported adherence to cardiovascular medications in resource-limited regions of the world.

Search Strategy

Our electronic search strategy included medical subject headings (MESH) and keywords related to medication adherence (e.g. “adherence”, “compliance”, “non-adherence”, “non-compliance”, “treatment refusal”), cardiovascular disease (e.g. “hypertension”, “hyperlipidemia”, “anti-diabetic”, “anti-atherosclerosis”), adherence measures (e.g. “medication monitoring”, “pill count”), cardiovascular medication classes (e.g. “ACE inhibitor”, “metformin”, “HMG CoA reductase inhibitors”, and “statins”), and resource-limited countries. Our list of resource-limited countries was based upon the International Monetary Fund list of “emerging and developing economies”, which include 153 countries in Africa, Southeast Asia, Eastern Europe, the Former Soviet states, Central and South America.7

Study Selection

Using pre-defined inclusion and exclusion criteria, two investigators (ADKB, JLL) independently reviewed the electronic search results to identify potentially relevant articles. Disagreements were resolved by consensus. We retrieved the published version of candidate articles and reviewed their reference lists to identify other studies that our search strategy may have missed.

We included studies that evaluated adherence to one or more cardiovascular medications. We excluded studies that: (1) did not present original data, (2) did not evaluate medications for the treatment of prevention of cardiovascular disease, (3) did not present quantitative adherence measures or (4) were not conducted in a resource-limited region. Included studies were not restricted to the English language and were translated accordingly.

Data Extraction

Data on patient and study characteristics, outcomes and study quality were independently extracted from each article by two investigators (ADKB, JLL) using a standardized protocol and reporting form. Specific information collected included study design (i.e. cohort, cross-sectional, randomized control trial), setting (i.e. country and rural or urban environment), patient demographics (including age and gender), the disease and drug evaluated and the method by which adherence was measured. Study quality was assessed with the Newcastle Ottawa Quality Assessment Scale8 for observational studies, the Agency for Healthcare Research and Quality (AHRQ)9 tool for rating cross-sectional studies and Jadad10 assessment for randomized control trials. A study quality score from each scale was calculated as a proportion of total points that each paper received. We also recorded information on predictors of adherence if any were reported.

Studies were categorized into four mutually exclusive categories based on the disease being treated: (1) diabetes, (2) hypertension, (3) congestive heart failure or (4) coronary artery disease. Studies that evaluated more than one disease (e.g. diabetes and hypertension) and presented these results separately were included in their appropriate category. Studies that did not report results disaggregated by disease sub-type or that did not specify the type of heart disease that patients had were included in the coronary artery disease category.

We also classified studies based on the method by which adherence was assessed: (1) pill counts, (2) self-report or (3) other. The latter category included studies that used electronic pill-bottles (e.g. medication event monitoring system [MEMS]), assessments by healthcare professional, reviews of health records and biochemical assays. In post-hoc analyses, we also evaluated subgroups based upon the complexity of medication regimens, the care setting, the use of drugs for primary as compared with secondary prevention, whether or not medications were provided to patients for free, age, gender and study quality.

Data Analysis

The main outcome measure of our study was a summary estimate of medication adherence. In order to pool studies, the variances of the raw proportions from individual studies (variance[r]) were stabilized using a Freeman–Tukey-type arcsine square root transformation: y = arcsine [√(r/n + 1] + arcsine[√(r + 1)/(n + 1)] with a variance of 1/(n + 1), where n represents the sample size of the study.11,12 A DerSimonian-Laird random effects method was then used to pool the transformed proportions.3,13,14 Our results are reported as summary estimates with 95% confidence intervals. All statistical analyses were conducted using SAS v9.2 (Cary, NC).

Between-study heterogeneity was explored in several ways. First, we visually inspected the plot of overall adherence proportions to look for outliers. Second, the proportion of the overall variation in adherence that was attributable to between-study heterogeneity was estimated with an I2 statistic.15 Third, heterogeneity was re-evaluated after influential studies were excluded. Finally, pooled adherence was calculated for each of our pre-specified study sub-categories. Pooling was only performed in subgroups with three or more studies.

Predictors of medication adherence were evaluated from those studies that reported empirical results about factors affecting adherence. Included studies either presented adherence rates stratified by a given predictor (e.g. men vs. women) or regression parameters (or correlation coefficients) for the association between adherence and a potential predictor. To maintain consistency across studies, predictors were reoriented, if necessary, to evaluate their association with rates of non-adherence rather than adherence. For example, if a study reported that lower medication costs were associated with higher rates of adherence, we report this as demonstrating a relationship between higher drug costs and higher rates of non-adherence. Because not all studies tested the statistical significance of the given predictor, we conservatively assumed that the associations of these predictors with adherence were not statistically significant.

RESULTS

Study Characteristics

Our search identified 2,353 abstracts, of which 76 studies met our inclusion criteria (Fig. 1). These studies included a total of 124,733 subjects (sample size range 17 to 100,691, median 157 subjects). Forty-nine studies evaluated adherence to antihypertensive medications1664 and an additional 17,23,55,6579 38082 and 98391 studies assessed medications for diabetes, congestive heart failure and coronary artery disease, respectively. The studies were predominantly performed in urban settings and were mostly based in Africa (40%), Asia (34%) or Central and South America (14%). All studies were either cross-sectional or cohort studies, with the exception of 5 randomized control trials. The majority assessed adherence using pill counts (n = 16) or self-report (n = 49). Further details of the study designs and patient demographics are presented in Table 1.

Figure 1
Flow diagram of study selection.
Table 1
Study Characteristics

Reported Adherence

The included studies reported adherence ranging from 0 to 98% (Fig. 2a-d). Only eighteen (23%) studies reported that, on average, patients were fully adherent to their prescribed therapy. Pooled across studies, overall adherence to cardiovascular drugs was 57.5% (95% confidence interval [CI] 52.3% to 62.7%; I2 0.98).

Figure 2Figure 2
a Adherence to medications for congestive heart failure. b Adherence to medications for diabetes. c Adherence to medications for coronary artery disease. d Adherence to medications for hypertension.

Subgroups

Reported adherence was relatively consistent across study subgroups (Table 2), although adherence to medications for congestive heart failure was lower (48.4%, 95% CI 9.0% to 89.2%; I2 0.68) than that for other disease categories. Studies using pill counts reported higher levels of adherence (62.1%, 95% CI 49.7% to 73.8%; I2 0.83) than those using self-report (54.6%, 95% CI 47.7% to 61.5%; I2 0.93) or other methods (63%, 95% CI 51% to 74.3%, I2 0.96) to estimate adherence. Adherence did not vary by geographic region or urban vs. rural settings, but when assessed in the context of randomized controlled trials, adherence was lower (42.6%, 95% CI 25.3% to 60.9%; I2 0.67) than in observational studies (59.0%, 95% CI 52.6% to 64.1%; I2 0.98). Similarly, adherence did not significantly change according to gender, age, the complexity of medication regimens, by clinical setting or the integrity of the studies (Table 2).

Table 2
Reported Adherence by Subgroup

Predictors of Adherence

Of the 76 papers included in our study, 29 reported factors associated with adherence. The most commonly and consistently reported predictors of non-adherence were poor knowledge (10 of 18 studies evaluating this factor reported a statistically significant association), negative perceptions about medications (11 of 15 studies evaluating this factor reported a statistically significant association), the occurrence of side effects (10 of 14 studies evaluating this factor reported a statistically significant association) and high medication costs (9 of 11 studies evaluating this factor reported a statistically significant association) (Fig. 3). All studies (n = 4) reporting social factors (e.g. lack of family support) as a predictor of non-adherence reported a significant association, as did the majority of studies (79%) evaluating a change (improvement or worsening) of symptoms. Patient factors such as age, gender, lifestyle factors, complex treatment regimens, and lack of access to health care services were not consistently associated with non-adherence. Restricting our analysis to studies that used significance testing to compare risk factors between adherent and non-adherent patients did not change our findings.

Figure 3
Factors predicting non-adherence to cardiovascular medication.

DISCUSSION

The role of medications in the management of cardiovascular disease is well recognized. While these conditions impose a greater burden in resource-limited than resource-rich countries, medication adherence in this context has received very little attention. Even the World Health Organization report1 which highlights the global problem of non-adherence relies almost exclusively on studies data from the developed world. To fill this void, we systematically reviewed studies in the peer-reviewed published literature that evaluated adherence to cardiovascular medications in the developing world. We found that although there was substantial heterogeneity across studies, overall adherence was 58%. This rate is remarkably similar to that observed in resource-rich regions.4,92,93 As such, our results highlight the quality improvement opportunity that exists worldwide from improving adherence to essential medications.

Given the scarcity of health resources available in resource-scarce countries, only quality improvement interventions that are cost-efficient are likely to be feasible.94 As a whole, increasing adherence to evidence-based medications is likely to be a more efficient strategy for improving cardiovascular outcomes than increasing treatment initiation rates or developing and evaluating new cardiovascular medications.95 Further, improved adherence has been shown to improve the effectiveness of interventions which target lifestyle modifications96 and may represent an opportunity to not only improve health quality but also reduce health care spending.1,2,4 This may be particularly true in resource-limited settings where the majority of cardiovascular medications are available as low-cost generic products.97

Unfortunately, the literature contains virtually no published reports of successfully implemented and rigorously evaluated cardiovascular medication adherence improvement strategies in resource-limited countries. Numerous strategies to improve adherence have been studied in the developed world. These include approaches that are “informational” (e.g. telephonic coaching, group classes, or the mailing of instructional materials), “behavioral” (e.g. pillboxes, mailed reminders, simplifying treatment regimens, or audit and feedback), “family and social focused” (e.g. support groups and family counseling), or some combination thereof.6

The studies we reviewed included a broad range of factors affecting adherence, with poor knowledge, negative perceptions about medications, the occurrence of side effects and high medication costs being evaluated most often and being most consistently associated with non-adherence. The literature evaluating reasons for non-adherence in resource-poor settings is extremely limited, and the most robust data comes from studies evaluating therapies for HIV. Mills et al. have found cost, complexity and perception of medications to be consistent reasons for non-adherence to medications in this context.6,98 These factors have also been observed in resource-rich settings as well.4,99 Thus, general approaches to non-adherence used in resource-rich settings may hold promise once translated into the developing world context.

We found adherence to be consistently poor across all of the disease subgroups we evaluated. The slightly worse adherence rates in studies of congestive heart failure medications may have been the result of the nature of the patient population or the severity of their disease, although these factors were not explored in any of the studies we evaluated. Interestingly, when assessed by pill count, adherence rates were better than when evaluated by self-report. This is somewhat different than studies in resource-limited settings where subjective measures tend to provide higher estimates of adherence than those provided by objective measures.92 While the reason for our apparently contrary findings are unclear, it may be that patients’ perceptions of medications and the social stigma associated with chronic disease may actually lead patients to under-report their true levels of adherence. Nevertheless, future adherence improvement in these resource-limited areas should pay particular attention to study design and the use of rigorous assessment methods.

Our study has several limitations. Although we have evaluated studies that have studied adherence rates in resource-rich countries, we did not directly compare adherence rates between the resource-rich and resource-limited countries, as no such studies exist. Although our search strategy included a wide range of electronic sources and our literature search sample was quite large, we may have missed some studies, especially if research conducted in resource-limited countries is less likely to be published. Furthermore, we did not include studies presented in abstract form at a scientific meeting but which were not subsequently published in the peer-reviewed literature. Due to the variation in trial size and methodology, there is significant heterogeneity between the studies, despite our having performed numerous subgroup analyses. It is possible that some of the between study differences in adherence we observed were due to differences in adherence patterns associated with different classes of medications to treat a single condition (for example, diuretics as compared to ACE inhibitors for hypertension).100 The included studies do not provide sufficient detail to explore this further. While our study summarizes possible predictors of non-adherence to cardiovascular medications, in some cases, these predictors were only reported by a minority of studies. As such, we are only able to comment on the importance of these factors as a proportion of studies actually report on them.

In conclusion, adherence to cardiovascular medication in resource-limited countries is sub-optimal and appears similar to rates observed in the developed world. Greater attention to long-term adherence in resource-limited countries should be a priority given the burden of heart disease in this context, the central role of medications in their management, and the clinical and economic consequences of non-adherence.

Acknowledgement

This work was not funded by any external sources.

Conflict of Interest None disclosed.

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