Additional tuberculosis case detection through repeat household contact investigation at 6 and 12 months: a retrospective cohort study in Kasama, a peri-urban district in Zambia

Background: Tuberculosis contact investigation (TBCI) facilitates early detection of TB among household contacts. However, evidence on the additional contribution of repeat TBCI in high-burden settings remains limited. We assessed the contribution of repeat TBCI at 6 and 12 months to TB case detection in Kasama District, Zambia. Methods: We conducted a retrospective observational cohort analysis of routinely collected programme data from five purposively selected high-volume TB diagnostic facilities. Household contacts of bacteriologically confirmed TB patients notified between January 2023 and April 2024 underwent symptom-based TB screening at baseline and at 6 and 12 months. We estimated round-specific TB detection yields, the cumulative proportion diagnosed with TB, and the contribution of repeat investigations to overall case detection. Results: Among 1,250 index TB patients, 3,648 household contacts were identified and screened at baseline. TB was diagnosed in 143 (3.9%) contacts at baseline, 31 (0.9%) at month 6, and 25 (0.7%) at month 12. Overall, 199 (5.5%) contacts were diagnosed with TB across the three screening rounds, of whom 159 (79.9%) had bacteriologically confirmed disease. Repeat investigations identified 56 additional cases, representing 28.1% of all TB diagnoses. Conclusions: Repeat TBCI at 6 and 12 months identified additional TB cases beyond baseline screening, although detection yields declined across successive rounds. These findings support further evaluation of risk-based follow-up strategies and more sensitive baseline screening approaches to optimize TB case detection and resource utilization in high-burden settings.