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Introduction: Increased equipment dead space relative to lung volume has been shown to impact multiple breath washout (MBW) outcomes (Benseler et al. Respirology, 2015). EcoMedics AG (Exhalyzer®D MBW device) recommends a weight-specific dead space reducer (DSR) to minimize burden in subjects with small lung volumes; subjects ≤35.0kg use DSR#2 (9.5mL dead space), subjects >35.0kg use DSR#3 (22mL dead space). We evaluated whether MBW outcomes are impacted in these subjects when using the recommended thresholds. Methods: Healthy children (HC) and children with cystic fibrosis (CF) performed MBW tests at three consecutive asymptomatic visits. At least two trials were obtained using both DSRs in random order at each visit. The difference in MBW outcomes between DSRs was compared using paired t-tests for each visit. Results: A total of 21 subjects (8 CF, 13 HC; 43% female) performed 56 paired measurements. The median (IQR) visits per subject was 3(2,3). Median (IQR) age at the initial visit was 9.1 years (8.4, 9.6) for HC and 10.3 years (9.5, 10.5) for CF subjects. Mean (SD) weight at first visit was 32.0 kg (2.1) and similar between groups. Lung clearance index was not systematically different between DSR#2 and #3 at first visit (Δ 0.14; 95% CI -0.05, 0.33; p=0.15). Measured functional residual capacity was lower using DSR#3 (Δ -0.09; 95% CI -0.14, -0.05; p Conclusion: Changing DSR at suggested weight threshold (35kg) does not systematically affect MBW outcomes. Funding: CFF and CF Canada