Predicting adherence to combination antiretroviral therapy for HIV in Tanzania: A test of an extended theory of planned behaviour model

Objectives: Combination antiretroviral therapy (cART) for HIV is widely available in sub-Saharan Africa. Adherence is crucial to successful treatment. This study aimed to apply an extended theory of planned behaviour (TPB) model to predict objectively measured adherence to cART in Tanzania.Design: Prospective observational study (n=158) where patients completed questionnaires on demographics (Month 0), socio-cognitive variables including intentions (Month 1), and action planning and self-regulatory processes hypothesised to mediate the intention-behaviour relationship (Month 3), to predict adherence (Month 5).Main outcome measures: Taking adherence was measured objectively using the Medication Events Monitoring System (MEMS) caps. Model tests were conducted using regression and bootstrap mediation analyses.Results: Perceived behavioural control (PBC) was positively (=.767, p<.001, R-2=57.5%) associated with adherence intentions. Intentions only exercised an indirect effect on adherence (B=1.29 [0.297-3.15]) through self-regulatory processes (B=1.10 [0.131-2.87]). Self-regulatory processes (=.234, p=.010, R-2=14.7%) predicted better adherence.Conclusion: This observational study using an objective behavioural measure, identified PBC as the main driver of adherence intentions. The effect of intentions on adherence was only indirect through self-regulatory processes, which were the main predictor of objectively assessed adherence.