Free HIV Antiretroviral Therapy Enhances Adherence among Individuals on Stable Treatment: Implications for Potential Shortfalls in Free Antiretroviral Therapy

OBJECTIVE: To estimate the population-level causal effect of source of payment for HIV medication on treatment adherence using Marginal Structural Models. METHODS: Data were obtained from an observational cohort of 76 HIV-infected individuals with at least 24 weeks of antiretroviral therapy treatment from 2002 to 2007 in Kampala, Uganda. Adherence was the primary outcome and it was measured using the 30-day visual analogue scale. Marginal structural models (MSM) were used to estimate the effect of source of payment for HIV medication on adherence, adjusting for confounding by income, duration on antiretroviral therapy (ART), timing of visit, prior adherence, prior CD4(+) T cell count and prior plasma HIV RNA. Traditional association models were also examined and the results compared. RESULTS: Free HIV treatment was associated with a 3.8% improvement in adherence in the marginal structural model, while the traditional statistical models showed a 3.1–3.3% improvement in adherence associated with free HIV treatment. CONCLUSION: Removing a financial barrier to treatment with ART by providing free HIV treatment appears to significantly improve adherence to antiretroviral therapy. With sufficient information on confounders, MSMs can be used to make robust inferences about causal effects in epidemiologic research.