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Introduction Dysfunctional breathing (DB) is a recognised contributor to breathlessness, occurring as the main cause or a contributing cause where other comorbidities exist. Cardiopulmonary exercise testing (CPET) can be used to assess for DB via pattern recognition by a CPET expert, but it has been suggested previously that numerical outputs could be used as an objective measure requiring less expertise.Aim To ascertain if there are objective measures from CPET outcomes that can be used to diagnose DB.Methods Retrospective CPET outcomes for patients referred to Respiratory Physiology between January 2020 and January 2025 were searched for using ‘DB’, Dys’, ‘breath’ and ‘SOB’ in the patient query program. Those without peak blood gas measurement or with known cardiac, vascular or ventilatory disorders were removed. Demographic data was taken from the patient’s records and CPET outcomes related to ventilation (peak VO2, VE/VCO2 slope, VE/VCO2eq at 40watts, PaCO2, PETCO2, respiratory rate (RR), P(a-ET)CO2, Vd/Vt, P(A-a)O2 gradient and breathing reserve (BR)) were compared to expert opinion using threshold values (30 or 35 for VEVCO slope depending on age, or 35 for VE/VCO2eq) or by analysing continuous data. Mann Whitney U, T-test and Chi square statistics were performed. ROC curve analysis was performed for VE/VCO2eq at 40w.Results 269 patient results were returned and 65 fulfilled inclusion criteria. Cardiovascular impairment was found in 25/65. 59% were diagnosed with DB, 39% of which had no other abnormalities. There were no significant differences in any variable between those with and without DB. Thresholds for VE/VCO2 slope and VE/VCO2eq at 40watts did not identify those with DB. ROC curve analysis gave an AUC of 0.632 for VEVCO2 at 40watts with a sensitivity of 63.2% and specificity of 63.0% using a threshold of 35. When only including those with DB alone, AUC reduced to 0.409 (40% sensitivity and 44% specificity) at a threshold of 35.Conclusions Individual CPET outcomes did not distinguish between those with and without DB in this cohort. Assessment by DB type (hyperventilation or breathing pattern disorder) alongside analysing combinations of CPET outcomes could be useful in a larger cohort.