Evaluation of normal pre-bronchodilator FEV1/VCMax in the context of significant bronchodilator responsiveness

<bold>Introduction:</bold> Bronchodilator responsiveness (BDR) during reversibility is possible in the absence airflow obstruction (AFO), and the varied definition of AFO and what constitutes a ‘significant’ BDR has led to differences in practice. This study aims to reaffirm the importance of considering reversibility in normality; and reopen debates as to whether performance of BDR tests should be limited to patients with defined AFO by considering a cohort of patients with ‘borderline’ AFO based on FEV1/VCMax standardised residuals (SR). <bold>Methods:</bold> 1142 BDR tests were assessed retrospectively. Analysis of BDR in patients with an FEV1/VCMax >-1.645 was performed (n=687, Female=420, Paediatric=53, Caucasian=677). Patients were stratified by FEV1/VCMax SRs into ‘borderline’ (−1.645<Z<-1.282) and ‘not borderline’ groups (Z>-1.282) and analysed against ARTP, ATS/ERS and NICE criteria. <bold>Results:</bold> BDR was most identified by ARTP 2020 guidance (n=92) (ATS/ERS (2021)=36, NICE (2019)=30 ATS/ERS (2005)=26). 89 patients (13%) were ‘borderline’, of these 23 (25%) showed significant BDR; 69 (12%) of the ‘not borderline’ group showed significant BDR. The highest FEV1/VCMax SR with significant BDR was 1.1, 95% of significant BDRs had an FEV1/VCMax SR < 0.1. <bold>Conclusion:</bold> ARTP (2020) guidance is the most lenient in determining a significant BDR, resulting in 92 (8%) of 1142 patients showing significant BDR despite a normal FEV1/VCMax. This highlights a significant patient group with potentially compromised diagnostic pathways if BDR tests were not performed. Applying a more inclusive FEV1/VCMax SR threshold may balance the proportion of this group against performing unnecessary BDR tests more effectively.