Schmidt A Yammine S Proietti E Frey U Latzin P Riedel T Singer F 2015 INTRODUCTION The new ATS/ERS consensus report recommends in vitro validation of multiple-breath inert gas washout (MBW) equipment based on a lung model with simulated physiologic conditions. We aimed to assess accuracy of two MBW setups for infants and young children using this model, and to compare functional residual capacity (FRC) from helium MBW (FRC(MBW)) with FRC from plethysmography (FRC(pleth)) in vivo. METHODS The MBW setups were based on ultrasonic flow meter technology. Sulfur hexafluoride and helium were used as tracer gases. We measured FRC in vitro for specific model settings with and without carbon dioxide and calculated differences of measured to generated FRC. For in vivo evaluation, difference between FRC(MBW) and FRC(pleth) was calculated in 20 healthy children, median age 6.1 years. Coefficient of variation (CV) was calculated per FRC. RESULTS In the infant model (51 runs, FRC 80-300 ml), mean (SD) relative difference between generated and measured FRCs was 0.7 (4.7) %, median CV was 4.4% for measured FRCs. In the young child model, one setting (8 runs, FRC 400 ml) showed a relative difference of up to 13%. For the remaining FRCs (42 runs, FRC 600-1,400 ml), mean (SD) relative difference was -2.0 (3.4) %; median CV was 1.4% for measured FRCs. In vivo FRC(pleth) exceeded FRC(MBW) values by 37% on average. CONCLUSIONS Both setups measure lung volumes in the intended age group reliably and reproducibly. Characteristics of different techniques should be considered when measuring lung volumes in vivo. https://sonar.ch/global/documents/363 eng info:eu-repo/semantics/altIdentifier/doi/10.1002/ppul.23010 info:eu-repo/semantics/altIdentifier/pmid/24574166 Pediatric pulmonology. - 2015 in vitro in vivo respiratory function tests validation studies Breath Tests Child Child, Preschool Female Functional Residual Capacity Humans Infant Lung Lung Volume Measurements Male Plethysmography Reproducibility of Results Respiratory Function Tests Tidal Volume Validation of multiple-breath washout equipment for infants and young children. http://purl.org/coar/resource_type/c_6501