Background The effects of a delayed diagnosis of asthma on lung function in children have not been well investigated. Therefore, a retrospective cohort study was conducted in a children’s hospital to analyse the effect of delayed diagnosis time on lung function in children with asthma. Methods We conducted a retrospective cohort study in Jinan Children's Hospital from January 1, 2010, to December 31, 2020. All children were divided into different groups according to the presence or absence of rhinitis, age at first onset (first coughing and wheezing attack) and delayed diagnosis duration (≤ 3 months, 3–12 months, 1–3 years, 3–5 years and > 5 years). Results A total of 1,014 children with asthma were included in this study. The median (quartile) delay in asthma diagnosis among all participants was 11 (2, 26) months. The shortest delay in diagnosis time was on the same day of onset, and the longest delay in diagnosis time was 10 years. The median (quartile) duration of delayed diagnosis was 10 (2, 26) months in 307 asthmatic children without rhinitis and 11 (2, 26) months in 707 children with asthma and rhinitis (P < 0.05). The delayed diagnosis time was shorter among female children than among male children (P < 0.05), and the first %predicted forced volume capacity (FVC%pred) results for females were higher than those for males (P = 0.036). The children whose age at first asthma onset was ≤ 3 years had a longer delayed diagnosis duration than those whose age at first onset was > 3 years (P < 0.05). The FVC%pred and %predicted forced expiratory volume in 1 s (FEV1%pred) in the first and second pulmonary function tests were significantly lower in the five delayed diagnosis groups (all P < 0.05). After standardised treatment for 3–6 months, FVC%pred showed a significant difference in the third test among the 5 groups (P < 0.05), but the other pulmonary function indices showed no significant difference. Logistic regression analysis showed that longer delay and young age of onset were associated with lower lung function (P < 0.05), whereas sex, rhinitis and eczema had no significant effects (all P > 0.05) on FVC%pred and FEV1%pred. Conclusion Although delayed asthma diagnosis can lead to lung function impairment in children with asthma, lung function can be improved quickly after standardised treatment. Therefore, early asthma diagnosis and standardised treatment are very important.

small airway indicators, which was lacking in previous studies [1][2][3]. Pediatricians should focus on changes in lung function as well as the clinical control of asthma. We also found that BMI, onset at 1-3 y of age, a longer time interval of delayed diagnosis, and a high allergy to dust mites were closely related to FEV1 decline. In particular, we have previously identified the impact of the delayed diagnosis on lung function [4], so the early diagnosis of asthma needs to be taken seriously.Funding This study was funded by the Jinan Key Laboratory of Pediatric Respiratory diseases (Xiang Ma is the PI), Clinical Science and Technology Innovation Plan in Jinan (202134067), Clinical Science and Technology Innovation Plan in Jinan ( 202225022), and Shandong Provincial Natural Science Program (ZR2021MH147). The funders had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication. DeclarationsConflict of Interest None.Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.