How many of persistent coughers have pulmonary tuberculosis? Population-based cohort study in Ethiopia.

Objective: Many individuals with persistent cough and smear microscopy-negative sputum test for tuberculosis (TB) remain at risk of developing the disease. This study estimates the incidence of pulmonary TB (PTB) among initially smear-negative persistent coughers and its risk factors.

Design: A prospective population-based follow-up study.

Setting: Health extension workers visited all households in Dale three times at 4-month intervals in 2016-2017 to identify individuals with symptoms compatible with TB (presumptive TB) using pretested and semistructured questionnaires.

Participants: We followed 3484 presumptive TB cases (≥15 years) with an initial smear-negative TB (PTB) test.

Outcome Measures: Bacteriologically confirmed PTB (PTB b+) and clinically diagnosed PTB (PTB c+).

Results: 3484 persons with initially smear-negative presumptive PTB were followed for 2155 person-years (median 0.8 years); 90 individuals had PTB b+ and 90 had PTB c+. The incidence rates for PTB b+ and PTB c+ were both 4176 (95% CI 3378 to 5109) per 100 000 person-years. We used penalised (lasso) and non-penalised proportional hazards Cox regression models containing all exposures and outcomes to explore associations between exposures and outcomes. In lasso regression, the risk of development of PTB b+ was 63% (HR 0.37) lower for people aged 35-64 years and 77% (HR 0.23) lower for those aged ≥65 years compared with 15-34 year-olds. Men had a 62% (HR 1.62) greater risk of PTB b+ development than women. The risk of PTB c+ was 39% (HR 0.61) lower for people aged 35-54 years than for those aged 15-34 years. Men had a 56% (HR 1.56) greater risk of PTB c+ development than women.

Conclusions: PTB incidence rate among persistent coughers was high, especially among men and young adults, the latter signifying sustained transmission. Awareness about this among healthcare workers may improve identification of more new TB cases.