Background <p>In China, most health facilities for Tuberculosis(TB) prevention and treatment are not TB-designated. Underdiagnosis in non-TB-designated health facilities(NTDHFs) is a major cause of under-detection and underdiagnosis of tuberculosis. The aim of this study is to assess the extent and patterns of TB underdiagnosis in these settings in China and to propose relevant improvement measures.</p> Methods <p>A retrospective analysis was conducted on patients with abnormal lung imaging who presented to non-TB-designated health facilities between January and June 2022. 100 NTDHFs were randomly selected. Panels of experts reviewed the diagnostic results of patients from NTDHFs based on the clinical information. The underdiagnosis of pulmonary tuberculosis(PTB), the diagnostic pathway, the performance of laboratory tests and the positive detection rate were subsequently analyzed.</p> Results <p>Among the 4138 patients, 426 were determined to be underdiagnosed with PTB. The overall underdiagnosis rate was 10.3% (95% CI: 9.4% − 11.2%), with the county/district institutions showing the highest rate(19.2%). In the entire study population, 36.8% underwent TB-related laboratory tests, with an underdiagnosis rate of 6.1%, and 29.5% underwent bacteriological tests, with an underdiagnosis rate of 5.6%. The three most commonly performed laboratory tests were sputum smear microscopy, molecular testing, and interferon-gamma release assay (IGRA), with positive detection rates of 21.6%, 47.1%, and 71.4%, respectively.</p> Conclusion <p>Approximately one out of ten PTB patients with abnormal lung imaging will be underdiagnosed in NTDHFs. This phenomenon is more prevalent in lower-level health facilities and non-respiratory departments. TB-related laboratory tests were not adequately performed in NTDHFs, particularly for asymptomatic patients.</p> Clinical trial number <p>Not applicable.</p>