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Background The Gold standard test for diagnosing sleep-disordered breathing (SDB) in paediatrics is polysomnography (PSG). In the UK, cardiorespiratory sleep studies (CRSS) are more common because of the labour intensive nature of PSG’s. However, a rising number of referrals and long ENT surgical waiting lists mean pulse oximetry is being increasingly utilised as a screening tool to triage patients requiring urgent surgical intervention and those who can wait. This case highlights the limitations of pulse oximetry (PO) in the diagnosis of SDB in certain instances.Case Summary A 3 year old female was referred to ENT due to blocked nose, nocturnal cough and snoring. Examination showed grade 2 tonsils, patent nostrils and normal ear canals. She was referred for a home oximetry study (table 1), which showed an ODI 4%: 121/hr. The patient had emergency MLB & tonsillectomy, and oximetry repeated 2 months post-surgery (table 1). Similar results were found, ODI 4%: 144/hr. An urgent inpatient CRSS was performed and this showed severe central sleep apnoea (CSA), cAHI: 110.7/hr with very little obstructive activity. Nocturnal oxygen therapy was commenced and the patient’s oxygen saturation profile normalised. An urgent MRI was performed, which showed a Chiari malformation and the patient required Foramen Magnum Decompression (FMD) in the hope this would improve her CSA. Nocturnal oxygen therapy continued for 15 months when a repeat CRSS showed significant improvement in central activity.Conclusion It is important to highlight the limitations of PO as a diagnostic tool in SDB. The unusual initial oximetry pattern should have prompted more detailed studies prior to ENT surgery. However, due to service pressures PO is being used more frequently, especially as a screening tool by ENT and district general colleagues.