Effect of isoniazid preventive therapy on tuberculosis incidence and its predictors among adult patients enrolled in antiretroviral therapy at public health facilities in Ambo Town, Oromia, Ethiopia: a retrospective cohort study

BACKGROUND: Tuberculosis is one of the major causes of illness and death among people living with the human immunodeficiency virus. The World Health Organization has recommended isoniazid preventive therapy for those patients to decrease tuberculosis-related infection and death. OBJECTIVE: To assess the effect of isoniazid preventive therapy on tuberculosis incidence and its predictors among adult patients enrolled in antiretroviral therapy at public health facilities in Ambo Town, Ethiopia, between 2016 and 2021. METHODS: An institution-based retrospective cohort study with a six-year follow-up from January 2016 to June 2021 was conducted from October 15, 2021, to November 10, 2021, among 771 (386 isoniazid-exposed and 385 non-isoniazid-exposed) adults on antiretroviral therapy at health institutions in Ambo Town. A simple random sampling technique was used to select patient records. Data were collected using a checklist and recorded on an electronic tool called 'Kobo-Collect' and exported to SPSS version 26.0 for further statistical analysis. Kaplan-Meier survival plots and the log-rank test were used to compare the crude effect in both the exposed and non-exposed groups on disease-free survival probabilities. Bivariable analysis was used to select candidates at a p-value < 0.25 and then entered multivariable Cox regression analysis to declare statistical significance at a p-value < 0.05 with the respective AHR. RESULTS: In this study, isoniazid preventive therapy has a 90.7% reduction effect on tuberculosis incidence (AHR = 0.093, CI = 0.029-0.31). The incidence of tuberculosis among the isoniazid-treated group was 0.2 per 100 person-years and 2.2 per 100 person-years in the non-isoniazid group. Regarding predictors, those who did not receive isoniazid (AHR: 8.9; 95% CI: 2.52-31.61), were at WHO stage 3 (AHR: 15.5; 95% CI: 6.55-30.47), had a CD4 count < 100 cells/µl (AHR: 4.33 (1.35-13.88)), had a body mass index < 18.5 kg/m² (AHR: 2.86, 95% CI = 1.59-15.16), and had no previous tuberculosis treatment (AHR: 95% CI: 18.0 2.18-48.57) were significant predictors of incidence of tuberculosis infection. CONCLUSION: This study found a relatively lower tuberculosis incidence, though several significant predictors were observed in those with advanced WHO stage, CD4 count < 100 cells/µL, no prior tuberculosis treatment, and low baseline body mass index, underscoring the importance of isoniazid preventive therapy, which should be prioritized with integrated clinical, nutritional, and preventive interventions to reduce tuberculosis risk. CLINICAL TRIAL NUMBER: Not applicable.