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 feel they have little control over their medication-taking behaviour. Interventions to improve attitudes about IST adherence and control of adherence behaviour are needed.