COVID-19 disruptions and spring surges: an interrupted time-series analysis of tuberculosis diagnostic challenges in Northeast Iran.

Tuberculosis (TB) remains a significant public health challenge in Northeast Iran, yet longitudinal data evaluating regional transmission patterns following COVID-19 disruptions remain limited. This cross-sectional study, conducted from 2017 to 2023 at Qaem University Hospital, Mashhad, Iran, analyzed 14,572 patients with suspected TB using smear microscopy, culture, and PCR to characterize the test positivity rate (TPR), temporal shifts, and diagnostic challenges. We identified a 10.3% TPR (1,494 out of 14,572), with significant demographic disparities: females accounted for 51.7\% of cases (OR = 0.74, 95\% CI: 0.665-0.824; p < 0.001), although the association strength was weak (Cramér's V = 0.05). Adults aged 65 and older represented 51.6\% of the cases. The COVID-19 pandemic led to a 33.3\% decline in diagnoses from 2019 to 2020, with outpatient recovery lagging behind inpatient services. Time-series analysis identified a significant structural break in March 2020 (p < 0.001), statistically confirming the sharp decline in diagnoses due to the pandemic. Bronchoalveolar lavage showed the highest positivity rate at 54.7\%, identifying 135 smear-negative/culture-positive cases. Seasonal peaks in spring (27.8\%) are hypothesized to result from post-winter Vitamin D troughs and social gatherings during Nowruz. These findings emphasize the importance of geriatric-focused screening, multimodal diagnostic protocols, and pandemic-resilient TB surveillance. Regional policies should focus on integrated respiratory screening and community-based interventions to reduce seasonal transmission.