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Introduction Sleep studies are recommended in children suspected to have severe obstructive sleep apnoea (OSA) to stratify perioperative risk before adenotonsillectomy and to avoid unscheduled high-dependency unit admission. The mean desaturation nadir (MDN) is thought to be a useful measure of the overall hypoxic burden from sleep disordered breathing (SDB) events. MDN values have been reported in healthy children but not in those with severe OSA or other categories of SDB.Methods We report the findings of a cross-sectional observational study in children with SDB assessed with multichannel sleep studies (MCSS) in a UK secondary care centre between July 2022 and November 2024. We evaluated data from 1031 children recorded in a sleep study database, which is maintained for audit and service evaluation purposes. Following exclusions, the data of 889 children were analysed for MDN values according to SDB categories. The parameters measured with MCSS include oximetry, video, sound, movement, heart rate and pulse transit time but do not include airflow or an Apnoea-Hypopnoea Index (AHI) which is reported with cardiorespiratory studies or polysomnography. SDB categories were determined using oximetry and video criteria.Results We found that MDN falls with increasing severity of SDB, most notably in children with moderate and severe OSA. Approximately 4% of the cohort (n=37) had severe OSA (defined using McGill’s criteria) and had a mean MDN of 88% (95% CI 87% to 89%).Conclusion Our findings suggest that MDN values <90% can identify children with severe OSA who may have increased perioperative risk.