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Background As obesity and the risk of obesity hypoventilation syndrome increase, accurate assessment of respiratory physiology is vital. Blood gas analysis remains central to this but concerns about sample reliability or ease of acquisition may contribute to inefficiencies, including repeated testing. This audit explores current practice in a tertiary sleep-respiratory centre as a baseline ahead of introducing v-TAC, a mathematically arterialised venous blood gas method shown to correlate closely with ABG PaCO 2 values.1Methods We reviewed blood gas sampling over two months (Aug–Sep 2024) in adult patients assessed or monitored for home NIV. Early repeat samples were used as a marker of clinician confidence in the results obtained. We recorded the number and type of blood gases (ABG, CBG, VBG) and the frequency of repeats within 15 minutes and 2 hours.Results Of 1,247 samples, 509 were CBGs, 479 ABGs, and 174 VBGs. There were 80 repeat samples within 15 minutes (6.4%) and 35 within 2 hours (2.8%). Repeats were most common with ABGs, likely due to perceived sampling or analyser error (e.g. suspected venous admixture). Median PaCO 2 values for ABG and CBG were 6.18 and 5.87 kPa respectively, showing a small but consistent bias, in keeping with previous matched data.1Discussion Nearly 9% of samples were repeated within short intervals, suggesting incomplete clinician confidence in the initial result. CBGs were repeated less often but demonstrated systematic underestimation of PaCO 2. While most patients with OSA do not require blood gas analysis, when performed it is essential that results are accurate and acceptable. These findings highlight the need for reliable, less invasive alternatives. Our audit provides a foundation for v-TAC implementation in the assessment of patients with suspected obesity-related hypoventilation.Reference Davies MG, et al. BMJ Open Respir Res 2023;10:e001537.