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

Abstract

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 (β = .767, p < .001, R2 = 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 (β = .234, p = .010, R2 = 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.