Predicting adherence to immunosuppressant therapy: a prospective analysis of the theory of planned behaviour

Background. The objective of this study was to increase the ability to predict renal transplant patients (RTPs) who are most likely to be non-adherent to their immunosuppressant therapy (IST). Methods. One hundred and fifty-eight RTPs completed questionnaires assessing Theory of Planned Behaviour (TPB) variables (attitudes, subjective norms and perceived behavioural control) relevant to intentions to adhere to their IST, with the addition of a general measure of past adherence to medical advice. In the full sample, intentions to adhere to IST was the outcome variable. In a subsample of 70 RTPs, the primary outcome was IST adherence. Results. TPB variables (attitudes, beta = 0.32, P < 0.01; perceived behavioural control, beta = 0.37, P < 0.01; but not subjective norms, beta = -0.001, ns) explained 41% of the variance in intentions to adhere to IST (P < 0.001). Past behaviour predicted perceived behavioural control (beta = 0.67, P < 0.001). Subsample analyses explained 33% (P < 0.001) of the variance in adherence, with intentions and past behaviour being the primary factors (P < 0.05). Conclusions. RTPs particularly at risk may be those who have a history of non-adherence to medical advice, especially when they have negative attitudes about IST adherence and feet they have little control over their medication-taking behaviour. Interventions to improve attitudes about IST adherence and control of adherence behaviour are needed.