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Introduction Forced Oscillation Technique (FOT) is a non-invasive method to investigate the mechanical properties of the respiratory system, it works by superimposing pressure signals over normal tidal breathing. In a previous study, we found no significant difference in FOT parameters across multiple breathing frequencies (BF) in healthy young adults (Howlett-Foster et al 2022). However, it is unclear whether this is also true for patients with obstructive lung disease. This study aimed to assess if a change in BF would affect FOT measurements in patients with a confirmed diagnosis of asthma.Method We recruited patients with evidence of obstructive lung disease (FEV1/FVC<LLN) from the severe asthma clinic at Cambridge University Hospital NHS Trust. They performed FOT (Resmon Pro Full – Restech Srl) at 3 different BF (15, 30, 40 breath per minute) in a randomised order for each participant, with measurements at each BF performed in triplicate. A metronome was used to ensure the correct BF was achieved. For normally distributed data we performed ANOVA tests and for non-normally distributed data Friedman tests to assess the significance of any differences in FOT parameters across the BF.Results We recruited 59 patients. The mean age of the population was 54.1 years (13.4), 58% were female, the median FEV1/FVC ratio z-score was -2.56 (-3.26, -1.96). As BF increased, from 15 to 40 bpm, there was a significant reduction (-12%) in total airway resistance (R5) (P<0.001). There was also a significant reduction (-72%) in Delta (R5-R19) (P<0.001), and an increase (14%) in reactance (X5) (P=0.081) ( figure 1). At higher breathing frequencies, 5% of study participants were re-classified as normal compared to 15 bpm when using z-scores for R5 (>1.645).Abstract O4 Figure 1Box plots displaying. (A) Change in Rrs at 5 Hz at different breathing frequencies; (B) Change in Rrs at 19 Hz at different breathing frequencies; (C) Change in R5-R19 at at different breathing frequencies; (D) Change in Xrs at 5 Hz at different breathing frequenciesConclusion Higher breathing frequencies significantly reduce total airway resistance, we recommend that when performing FOT, BF should be controlled at approximately 15 bpm to reduce risk of misclassification.