Context-dependent risk stratification using a tuberculin skin test induration threshold of ≥ 15 mm in school tuberculosis screening in Hangzhou, China: a historical cohort study.

BACKGROUND

Larger tuberculin skin test (TST) reactions are associated with higher tuberculosis (TB) risk, but the operational value of the ≥ 15 mm threshold among adolescents who already have a TST induration ≥ 10 mm during routine school screening is less well characterised in high-BCG coverage settings. We assessed whether TST induration ≥ 15 mm provided incremental risk stratification information for secondary school students and interpreted this threshold across routine screening, outbreak contact and previously cured TB contexts.

METHODS

We conducted a historical cohort study using school-based TB screening and TB management data from Yuhang District, Hangzhou, China. The primary cohort included 6077 students aged 12-18 years who were screened between September 2023 and November 2025 with a baseline TST induration ≥ 10 mm and no active TB at screening. Students were classified as having a TST ≥ 15 mm or a TST 10-14 mm. The incidence of active pulmonary TB was ascertained by deterministic linkage with TB management systems. Students in the 2020-2022 TST ≥ 15 mm routine-screening descriptive cohort, school-outbreak contacts and previously cured TB patients were analysed descriptively and were not included in the primary inferential comparison.

RESULTS

In the primary cohort, four incident active pulmonary TB cases occurred among students with TSTs ≥ 15 mm (0.25%; 169.03 per 100 000 person-years), whereas none occurred among students with TSTs 10-14 mm. The adjusted incidence density ratio was 22.74 (95% CI 2.42-3015.62; P = 0.004), with wide uncertainty due to sparse events. The 2020-2022 TST ≥ 15 mm routine-screening descriptive cohort had a similarly low absolute incidence (128.64 per 100 000 person-years). In the outbreak contact cohort, the incidence among TST ≥ 15 mm contacts was 438.68 per 100 000 person-years; this estimate was interpreted descriptively because baseline definitions and preventive-treatment coverage differed.

CONCLUSIONS

A TST induration of ≥ 15 mm was associated with a higher incidence of active pulmonary TB in students than a TST of 10-14 mm, but the absolute risk after routine screening remained low. The threshold should be interpreted as a context-dependent risk marker that can support, but should not replace, the assessment of exposure history, previous TB and time since screening.