Asymptomatic tuberculosis in Japan: a 15-year national surveillance study, 2008–2022

Background: Asymptomatic tuberculosis (TB), defined as active TB without reported symptoms, remains poorly characterised. We assessed long-term trends, factors associated with asymptomatic diagnosis, and treatment outcomes of asymptomatic TB in Japan using nationwide surveillance data with widespread chest radiographic screening. Methods: We analysed bacteriologically confirmed pulmonary TB cases reported to the Japan Tuberculosis Surveillance System between 2008 and 2022. Patients were classified as asymptomatic, respiratory symptomatic, or non-respiratory symptomatic TB. Trends in symptom status were assessed. Factors associated with asymptomatic diagnosis and mortality during treatment were evaluated using modified Poisson regression with cluster-robust standard errors among cases notified during 2017-2022, when key covariates were consistently available. Findings: Among 141,481 pulmonary TB cases, 20.2% were asymptomatic, 61.3% had respiratory symptoms, and 18.5% had non-respiratory symptoms. Over time, the proportion of asymptomatic TB increased slightly, remaining around one-fifth of all pulmonary TB cases and 12-15% of cases with smear-positive or cavitary disease. Among asymptomatic patients, 34.1% were smear positive and 20.3% had cavitary disease. In multivariable analysis, asymptomatic TB was more common in younger groups, those in employment or education, and those with previous TB treatment. Mortality during treatment was lower in asymptomatic TB (adjusted risk ratio 0.57, 95% CI 0.54-0.60) than in respiratory symptomatic TB. Interpretation: Asymptomatic TB represents a substantial and persistent component of the TB burden in Japan, including potentially infectious cases. Reliance on symptom-based case detection alone may miss a considerable proportion of TB, supporting the need for targeted screening and earlier diagnosis. Funding: Research Institute of Tuberculosis (general research funds).