Global Lung Function Initiative reference values for multiple breath washout indices

Background Multiple breath washout is a lung function test based on tidal breathing that assesses lung volume and ventilation distribution. The aim of this analysis was to use the Global Lung Function Initiative methodology to develop all-age reference equations for the multiple breath washout indices lung clearance index (LCI) and functional residual capacity (FRC). Methods Multiple breath washout data from healthy individuals were collated from sites. Data were re-analysed using the latest software versions. Reference equations were derived using the lambda-mu-sigma method using the generalised additive models of location shape and scale programme in R. The impact of equipment type, inert tracer gas and equipment dead space volume on the derived reference ranges were investigated. Results Data from 23 sites (n=3647 test occasions) were submitted. Reference equations were derived from 1579 unique observations from participants between the ages of 2 and 81 years. Equipment type, inert tracer gas and equipment dead space volume did not significantly affect the prediction equations for either LCI or FRC. Reference equations for LCI include age as the only predictor, whereas sex-specific reference equations for FRC included height and age. Conclusions Global Lung Function Initiative reference equations for multiple breath washout variables provide a standard for reporting and interpretation of LCI and FRC.