Background
Theories may assist in the design of behaviour change interventions in various ways [
1‐
3], by promoting an understanding of health behaviour, directing research and facilitating the transferability of an intervention from one health issue, geographical area or healthcare setting to another.
Ensuring treatment adherence presents a considerable challenge to health initiatives. Haynes et al. ([
4], p2) have defined adherence as "the extent to which patients follow the instructions they are given for prescribed treatments". Adherence is a more neutral term than 'compliance', which can be construed as being judgmental. While programmes promoting adherence have focused on various health behaviours, this review focuses specifically on long-term adherence to tuberculosis (TB) and HIV/AIDS treatment. Non-adherence to treatment for these diseases has severe human, economic and social costs. Interrupted treatment may reduce treatment efficacy and cause drug resistance [
5], resulting in increased morbidity and mortality and further infections. Without intervention, adherence rates to long-term medication in high income countries are approximately 50% [
6], while adherence in low and middle income countries may be even lower [
7].
TB and HIV present particular challenges to adherence. Both are chronic and infectious diseases that affect mainly the most disadvantaged populations and involve complex treatment regimens with potentially severe side effects; both are public health priorities and non-adherence may cause drug resistance [
7]. These characteristics differentiate these diseases from other chronic diseases such as asthma and hypertension where, for example, drug resistance is not a key issue. Treatment adherence is also affected by beliefs about the origins, transmission and treatment of TB and HIV, often resulting in the stigmatisation of those affected [
7]. The interaction of these factors make adherence for these diseases not only a priority but a complex health issue.
Various interventions have been designed to improve treatment adherence, but few theories describe specifically the processes involved. Currently, there are more than 30 psychological theories of behaviour change [
8], making it difficult to choose the most appropriate one when designing interventions. This is a particular problem within the field of adherence to long-term medications, where the consequences of non-adherence may be severe. Existing theories therefore need to be examined further to determine their relevance to the issue of long-term medication adherence.
Leventhal and Cameron [
9] identified five main theoretical perspectives related to adherence: 1) biomedical; 2) behavioural; 3) communication; 4) cognitive; and 5) self-regulatory. Each perspective encompasses several theories. More recently, the stage perspective has emerged, which includes the transtheoretical model. The most commonly used theories are those within the cognitive perspective [
1,
10] and the transtheoretical model [
1]. This review includes a short description of theories within each of the five perspectives listed above, as well as the transtheoretical model. We locate these theories specifically within the realm of adherence to long-term medication, defined as medication regimens of three months or more; describe their key characteristics and evidence base; and examine their relevance and applicability with regard to adherence to long-term medication regimens for TB and HIV/AIDS. To our knowledge, the area of long-term adherence to medication has not yet been addressed in reviews of health behaviour theories.
While the focus of this review is on factors affecting consumers, we acknowledge that adherence is a complex and dynamic phenomenon, which relates to consumers, providers, health systems and broader socio-economic and political contexts. Although the theories chosen for this review focus mainly on providers and consumers, this is not the only area in which adherence can be promoted. The review is intended as an information source for those wishing to develop theory-based interventions focusing on intra- or interpersonal factors to increase TB and/or HIV treatment adherence.
Methods
A search was performed on MEDLINE, CINAHL, Pre-CINAHL, PsycInfo, ScienceDirect and ERIC databases using the keywords 'health and behaviour and (model or theory)'; '(model or theory); (adherence or concordance or compliance)', from the start date of each database to February 2005. Additional searches were performed in the University of Cape Town library, Google and Google Scholar. Citations were also identified from included papers. Finally, all databases consulted were searched again using the names of theories as keywords, with 'meta-analysis' or 'systematic review' in April 2005. Experts were consulted for comments and references. Published articles or book chapters in English, describing a particular theory, and articles that presented a meta-analysis of the theory, were included. Articles were excluded if they did not satisfy the aforementioned criteria. Where possible, interventions related to TB or HIV adherence were identified. No authors were contacted. Several additional randomised controlled studies or other articles were also included as examples of the use of theories in intervention development. In this paper we use the term 'theory', instead of 'model', and the term 'variable', instead of 'construct', when referring to a part of the theory.
Discussion
This review has discussed a number of health behaviour theories that contribute to understanding adherence to long-term medications, such as those for TB and HIV/AIDS.
Although the use of theory to develop interventions to promote adherence offers several advantages, it also has some limitations. Firstly, there is little evidence that allows for the direct comparison of these theories [
66]. Combining studies based on even one theory, in order to perform a meta-analysis to assess its effectiveness in predicting behaviours, is difficult due to various methodological problems in the original studies [
60]. Furthermore, the number of theories in this field has proliferated over time, as theorists have examined different areas of behaviour and engaged in re-examining existing explanatory theories. Researchers, health planners and practitioners may therefore be overwhelmed by the multitude of theories available to them and the fragmented, and often contradictory, evidence. Questions also remain regarding the applicability of these theories to contexts other than those in which they were developed. Ashing-Giwa [
67], for example, suggests that the above theories do not address socio-cultural aspects sufficiently. Issues such as the stigma attached to TB due to its perceived relation to HIV (especially in developing countries) may impact on the acceptability and the uptake of interventions. Further attention should therefore be given to the question of whether theories developed in the USA and the UK are applicable to individuals in other contexts where the disease burden from HIV/AIDS and TB is greatest.
Secondly, health behaviour change theories have tended to encompass a wide variety of health behaviours, each qualitatively different. The systematic reviews identified for this paper included studies ranging from smoking cessation to mothers limiting babies' sugar intake. Particular theories may be more applicable than others to improving adherence to specific health behaviours. For example, adherence to long-term medication will necessarily be different to a behaviour change required to take up exercise. In addition, achieving adherence to TB medication may be seen as an urgent issue for public health because of its infectiousness, and the recent emergence of extremely drug resistant strains [
68]. It is difficult therefore to compare the effects of the theories across health categories or even within individual categories.
Thirdly, few studies were identified that had examined the selected health behaviour theories in relation to long-term medication adherence, or that had developed interventions to promote long-term adherence explicitly based on these theories, particularly for TB. Sumartojo's [
13] assessment that a theory-based approach has largely been absent within the field of TB behavioural research appears to remain valid today.
The application of theories to the design of interventions remains a challenge for researchers and programme planners [
69] and there is considerable debate concerning the effectiveness and usefulness of theory in informing intervention development (see [
2,
70,
71]). Despite a variety of studies in a variety of fields, or perhaps because of this variation, we would argue that there is no clear evidence yet for the support of any of these theories within the field of adherence behaviours. This is not to say that these theories cannot be useful – rather, we have insufficient evidence to conclusively determine this.
While these discussions continue, research should aim to shed light on the key questions related to the theory-intervention debate: Do sound theories result in effective interventions? Does an effective intervention constitute proof of a theory's value? How might theory be used to inform the design of an effective intervention? And how can a theory be reliably tested? Some research work has already been undertaken in these areas: in a systematic review of antiretroviral treatment adherence interventions, Amico et al. [
72] found that the use of theory in constructing an intervention did not account for variability in the intervention's efficacy. However, it is unclear how many of the 24 included studies in this review articulated a health behaviour change theory or the extent to which this was done.
Two possible approaches have been suggested to addressing the difficulties raised by the multitude of existing theories on health behaviour change. One approach is to attempt to identify variables common to these theories. This has been undertaken for 33 health behaviour change theories [
7] in order to make psychological theories more accessible and easier to select. The results of this study provide some guidance on the most important variables in psychological theories, and may assist in the further development of health behaviour change theories. A second approach is to attempt to integrate the theories. While there is a need for such theoretical integration [
73], we argue that researchers and theorists alike should be cautious when picking and choosing parts of other theories to develop further theories – so-called "cafeteria-style theorizing" – as the resulting theories may include redundant variables [[
29], p. 285].
Because some theories share overlapping variables describing using different names [
8,
41], and most differences are due to an emphasis of one variable over another [
1], it would serve the development of this field to conduct studies to identify particular variables that perform best in predicting behaviour change. For example, in a meta-analysis of randomised controlled trials testing antiretroviral treatment adherence interventions, Simoni [
16] found that giving basic information to patients, and engaging them in discussion about helping them to overcome cognitive factors, lack of motivation and unrealistic expectations about adherence, were effective in improving adherence. Similarly, comparative studies between theories could be used to identify effective components [
74]. The field of health behaviour theory remains dynamic, and it is important to continue developing existing theories and approaches as new evidence emerges.
Applying health behaviour theories to medication adherence for TB and HIV/AIDS
How optimal adherence for TB and HIV/AIDS can be ensured remains an important question. While large numbers of studies have explored patients' and health care providers' views regarding adherence to TB treatment [
75] or have described programmes to improve adherence to these medications, there are still relatively few rigorous evaluations of interventions to promote adherence to TB and HIV/AIDS treatments [
76,
77]; even fewer have explicitly utilised behaviour change theories. For example, a systematic review of interventions to promote adherence to TB treatment [
77] included ten trials, none of which used an explicit theoretical framework. A similar review identified seven different randomised controlled trials of interventions to promote adherence to antiretroviral therapy [
76], of which only one employed an explicit theoretical framework. Similar figures have been reported in other domains: a review of guideline implementation studies showed that less than 10% of these provided an explicit theoretical rationale for their intervention [
78]. Given the paucity of evidence to support any particular health behaviour theory, we cannot therefore suggest that these theories be used routinely to design adherence promoting interventions. However, since these theories may well have practical behaviour change potential, and since the problem of medication adherence remains significant for both clinical medicine and public health, further exploratory and explanatory research is needed.
A number of recommendations emerge from this review (Table
2): firstly, future research should focus not on the development of new theories but rather on the further examination of those already elaborated. Several key attributes that should be encompassed by theories explaining behaviour change have been suggested, including demonstrated effectiveness in predicting and explaining changes in behaviour across a range of domains; an ability to explain behaviour using modifiable factors; and an ability to generate clear, testable hypotheses. The theories should include non-volitional components (i.e. issues over which individuals do not have complete control) and take into account the influence of external factors, as perceived by individuals [
2,
70].
Table 2
Recommendations for using health behaviour theories to develop long-term adherence promoting interventions in TB and HIV
▪ Future research should focus on the further examination of existing theories. |
▪ Further work is required to identify and explore health behaviour theories most applicable to improving adherence to long-term medications. |
▪ Existing health behaviour models should be tested systematically. |
▪ Interventions utilising health behaviour theories appropriately need to be developed and trialled. |
▪ Reports of interventions to promote adherence to long-term medications for other health issues should be reviewed to explore how these have drawn on health behaviour theories. |
Secondly, further work is required to identify theories of health behaviour that are most applicable to improving adherence to long-term medication. Existing health behaviour theories should be tested systematically to establish which best predict effects on different kinds of behaviour for different groups of people in different contexts. For example, does a particular theory predict changes in adherence behaviour for both men and women with TB in both England and South Africa? Some researchers have argued that experimental research and increased clarity in theories and methods could assist in the identification of effective behaviour change techniques, thereby contributing to the development of evidence-based practice in health psychology and implementation research [
2,
3]. Similar efforts need to be made regarding the use of theories as applied to adherence behaviour.
Thirdly, the abundance of theories and their poor evidence base highlights the need to develop and trial interventions that utilise these theories appropriately (i.e. in concordance with the theory), with well defined and operationalised variables. This will help to advance the study of human adherence behaviour and allow for better informed decisions related to how to these theories could be more widely applied in practice. (See references [
2] and [
75] for guidance on developing theoretically informed interventions). We have compiled a number of examples [see additional file
1] of the application of such theories in practice.
Finally, reports of interventions to promote adherence to long-term medications for other health issues, such as diabetes, asthma and hypertension, should be reviewed to determine how many have drawn on theory in the design and testing of these interventions; the range of theories utilised and the ways in which this was done; and the ways in which the use of theory contributed to understanding the effects of these interventions. Many reviews of such interventions exist (for example, see [
83,
84]) and these could act as a starting point for such work.
It is also important to list some of the limitations of this review. Firstly, we have been unable to capture all the available data on tests of health behaviour theories. Secondly, this paper examines only theories constructed by researchers and does not explore the health theories held by those receiving treatment. These lay theories of adherence with regard to antiretroviral [
81] and TB treatment [
75] are discussed elsewhere.
It should also be noted that any understanding of individual health behaviour, and interventions to change this, must be located within the relevant social, psychological, economic and physical environments [
28]. Much research on adherence to TB medication has indicated that poor adherence is commonly the result of factors outside the individual's control, including clinic and health care organisation factors (such as interruptions to drug supply and long distances to health facilities) and structural factors (such as poverty and migration) [
13,
82,
83]. Similar issues have been reported for adherence to ART [
84]. Any focus on changing the behaviours of individuals with TB or HIV should not result in the neglect of these other dimensions or the further disadvantaging of the poor and vulnerable, thereby widening health disparities. Interventions that focus on providers, the provider-patient relationship, health system and contextual factors therefore also need to be developed and evaluated [
76].
Competing interests
The author(s) declare that they have no competing interests.
Authors' contributions
JV and SL developed the idea for this paper, SM performed all searches and compiled the text, SL contributed to the writing and SL, TM and JV provided conceptual and editorial input. All authors read and approved the final manuscript.