Treatment effectiveness is determined jointly by the efficacy of the treatment agent and the extent of adherence to the treatment. Despite the availability of efficacious interventions, nonadherence to treatment remains a problem across therapeutic areas.
Adherence is a complex behavioural process determined by several interacting factors. These include attributes of the patient, the patient's environment (which comprises social supports, characteristics of the health care system, functioning of the health care team, and the availability and accessibility of health care resources) and characteristics of the disease in question and its treatment.
There are many specific aspects of treatment to which a patient may not adhere, for example:
- health-seeking behaviours (such as appointment-keeping);
- obtaining inoculations;
- medication use (use of appropriate agents, correct dosing and timing, filling and refilling prescriptions, consistency of use, duration of use); and
- following protocols for changing behaviour (examples include modifying diet, increasing physical activity, quitting smoking, self-monitoring of symptoms, safe food handling, dental hygiene, safer sex behaviours and safer injection practices).
The most frequently cited conceptual definition of adherence is "the extent to which a person's behaviour - taking medication, following a diet, executing lifestyle changes - follows medical advice" (26). Adherence has also been defined as "the extent to which patient behaviour corresponds with recommendations from a health care provider" (27,28). It has also been suggested that a more practical approach is to define adherence as "following treatment at a level above which treatment goals are likely to be met". However, these broad definitions belie the complexity of the issue.
In research, adherence has been operationalized in many different ways: as the degree to which a regimen is followed expressed as a percentage or ratio, a categorical phenomenon (e.g. good versus poor adherence), or as an index score synthesizing multiple behaviours. However, for clinical purposes, these definitions lack specificity, and give no clear direction for assessment and intervention.
The treatments that patients are asked to follow vary according to the nature of the demands they impose. They range from requiring relatively simple and familiar behaviours, to more complex and novel ones. Some treatments involve one behaviour, while others carry multiple behavioural requirements. Protocols also vary in terms of the length of time for which they must be followed. This means that the nature and meaning of adherence change according to the specific treatment demands of a particular protocol. Assessment and intervention strategies will differ according to the circumstances and/or intensity of the recommendations. All treatments make demands of one type or another on patients. Patients differ in their ability to meet those demands, and the resources available and the environmental contexts outlined earlier also differ. Perhaps adherence might be better understood as reflecting the process of efforts, occurring over the course of an illness, to meet the treatment-related behavioural demands imposed by that illness. This behavioural conceptualization allows us to define adherence more explicitly according to the type of behaviour, an acceptable frequency, consistency, intensity and/or accuracy.