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Background Many services are using wearable technologies with automated scoring algorithms to meet growing demand for OSA screening, including bariatric surgery candidates, who have a fundamentally different physiology. WatchPAT has been validated against polysomnography but studies have not specifically focussed on bariatric patients. This study aimed to compare AHI from WatchPAT 300 to respiratory polygraphy (RP, Embletta MPR) in patients undergoing bariatric surgery and sleep clinic controls.Methods Patients referred on the bariatric pathway for pre-operative screening of OSA were eligible to take part. A comparator group comprised of GP referrals with suspected OSA (BMI < 35 kg/m2). Exclusion criteria were use of medications or conditions known to affect the PAT signal (long-acting nitrates, alpha blockers, sympathectomy). Patients wore RP and WatchPAT concurrently for one night. RP signals were manually scored to produce an apnoea hypopnoea index (AHI) with start and stop times aligned to the time of sleep onset and final awakening determined by WatchPAT. WatchPAT AHI was automatically generated. Primary outcome measure was difference in AHI (WatchPAT-RP) and was compared statistically with Mann-Whitney U.Results 85 patients (53 bariatric) were included in the final dataset. Bariatric patients had a higher BMI than sleep clinic referrals (mean ± SD 46.52 ± 9.06 versus 28.62 ± 4.02 kg/m2, p=0.000) and had a greater percentage of females (77 versus 34%). The groups were similar in age (43.23 ± 11.61 versus 45.53 ± 11.98 years, p=0.391). Median (range) AHI was higher in WatchPAT compared to RP in both groups [bariatric: 21.8 (3.9–103.8) versus 9.60 (0–115.7) events/hour, p=0.000; controls 26.7 (2.3–81.5) versus 9.25 (0.3–73.3) events/hour, p=0.000]. Classification of severity revealed 43% of the bariatric group and 57% of all patients classed as having a normal or mild AHI on RP were classed as moderate or severe on WatchPAT.Conclusions Demand for pre-operative screening for OSA constitutes a significant part of the workload for sleep clinics. CPAP is often prescribed peri-operatively for bariatric surgery patients with at least moderate OSA. Further research is needed to understand the reasons for the differences in AHI between RP and WatchPAT and to determine the clinical significance.