Predicting adherence to combination antiretroviral therapy for HIV in Tanzania using an extended TPB model

Background: Combination antiretroviral therapy (cART) for HIV is now widely available in sub-Saharan Africa. Adherence is crucial to successful treatment. There is currently a lack of high-quality, theory-informed evidence identifying the psychological predictors of adherence in this setting. This study aimed to apply an extended Theory of Planned Behaviour (TPB) model to predict objectively measured adherence to cART among Tanzanian patients. Methods: 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). Taking adherence (% of prescribed doses taken) was measured objectively using the Medication Events Monitoring System (MEMS) caps. Mokken scaling analyses confirmed the TPB factor structure. Model tests were conducted using regression and bootstrap mediation analyses.Findings: Perceived behavioural control (PBC) was positively (β=.798, pu003c.001) and outcome expectancies were negatively (β=-.163, pu003c.01 ) associated with adherence intentions. Intentions did not predict behaviour directly, but exercised an indirect effect on adherence (B=1.34 [0.339 to 3.28]) through self-regulatory processes (B=1.11 [0.122 to 2.98]). Self-regulatory processes (β=.238, p=.010) were the only significant predictor of better adherence. Discussion: This observational study, using an objective behavioural measure and psychometrically sound scales, identified PBC and self-regulatory processes as the main drivers of adherence (intentions).