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Introduction Anaerobic threshold (AT) measured during a cardiopulmonary exercise test (CPET) is often considered as a percentage of the peak predicted V̇O2 (AT/peak predicted VO2), with a ratio less than 40% indicating disease (Cooper and Storer 2001). Knox-Brown et al (2025) recently validated this in an adult population referred for shortness of breath, with a threshold below 44% indicative of disease. We aimed to investigate the appropriateness of these criteria in a paediatric, cystic fibrosis population.Methods An incremental ramp CPET was performed on a cycle ergometer (Vyntus CPX, Jaeger Medical). Receiver Operating Characteristic (ROC) analysis with Area Under the Curve (AUC) was used to determine the threshold with the optimal sensitivity and specificity for AT/peak predicted VO2 that best discriminated a low peak VO2 (<LLN, Blanchard 2018). For comparison, we repeated this analysis using the AT/measured peak VO2.Results Data from 40 CF patients with a physiologically maximal exercise test were included. Median age was 13 years (12-14), 20% (8 of 40) were female, and median peak VO2pp was 88.5pp. Both expressions of AT gave an optimal threshold of approximately 50%. However, for AT/peak predicted VO2 this threshold was significantly better at discriminating a low peak VO2 (table 1).Conclusion AT/peak predicted and AT/measured VO2 are not interchangeable methods of indicating disease in a paediatric, cystic fibrosis population. BMI was found to be a likely explanation of this difference. Current thresholds used in adults to define deconditioning/disease are likely not appropriate in paediatrics and further research in a large sample is recommended.Abstract P47 Table 1Optimal thresholds analysisOptimal thresholdSensitivitySpecificityAUC (95%CI)AT/peak predicted VO2 <50.4% 91% 44% 0.70 (0.54 -0.87) AT/measured peak VO2 <50.2% 73% 56% 0.61 (0.43-0.79) p-value for difference <0.001