Introduction and objectives Shortened multiple breath washouts (MBW) are attractive in young children, where a long test time may not be tolerable (Thorax 2013;68:586–7). Lung clearance index (LCI) is usually calculated at 1/40^th of the starting concentration (LCI_std), but in shorter MBWs the LCI is calculated at 1/20^th of the starting concentration (LCI_0.5). LCI_0.5 and LCI_std are closely correlated but not interchangeable, and so we calculated an extrapolated full LCI (LCI_ex) from LCI_0.5. Our hypothesis was that extrapolated LCI (LCI_ex) will better reflect LCI_std than LCI_0.5 and be more sensitive to intervention.

Methods Condition-specific equations for cystic fibrosis (CF), primary ciliary dyskinesia (PCD) and asthma, and an overall equation for all 3 groups, were developed to convert LCI_0.5 to LCI_ex from data previously analysed (n = 90, Thorax 2014;69[Suppl²]A166). LCI_std, LCI_0.5 and LCI_ex were then calculated for new cohorts (n = 70, 20 asthma, 30 CF, 20 PCD), and LCI_ex was compared to LCI_std. CF patients receiving IV antibiotics (n = 17) and asthma patients receiving triamcinolone (n = 32)) also had LCI_std, LCI0.5 and LCI_ex calculated and compared. The upper limit of normal for LCI_std was calculated from healthy controls. LCI_ex was compared to LCI_std with a Bland-Altman plot.

Results In CF, at higher LCIs, the spread between LCI_ex and LCI_std grew but there was no bias. For PCD and asthma agreement remained very good. There was no significant difference between LCI_std and LCI_ex. Results for positive and negative prediction of LCI_std for LCI_0.5 and LCI_ex are shown in the Table 1. LCI_ex and LCI_0.5 were also sensitive to interventions in asthmatic and CF patients, although in general LCI_std had better p values.

Conclusions LCI_ex performed better than LCI_0.5 in predicting results of LCI_std. However, LCI_ex did not always reflect LCI_std, particularly in CF, so the two cannot be used interchangeably in lung disease.