Validation of multiple-breath washout equipment: from bench to clinic and possible pitfalls.

much more widely used, such as spirometry, body plethysmography or T L CO . Before results of MBW can be compared between centres or even multi-centre trials can be performed, these kinds of validation studies are extremely important to guarantee comparability and identify sources of errors. In this regard, Gonem et al. [15] are to be congratulated for the thorough validation study of the Innocor system published in this issue of Respiration . The authors do not hesitate to report openly the current drawbacks of the equipment and nicely show that functional residual capacity (FRC) underestimation at low lung volumes will result in a very relevant overestimation of LCI at those low lung volumes [15] . These results are surprising as they oppose the previous assumption that LCI might be more robust to measurement errors compared to FRC, as LCI is a ratio of lung volumes and possible measurement errors might cancel out. This is not the case, and the measured overestimation in LCI is clinically important. Moreover, this type of error may be problematic when using the same equipment in patients with a wide age range, leading to incorrect physiological conclusions based on possible technical issues [16, 17] . In recent years, there has been a renaissance of studies using multiple-breath washout (MBW) tests in different populations with varying lung diseases. To date, measures obtained from the MBW have followed two parallel paths. First, the lung clearance index (LCI) has been shown to be a sensitive marker of cystic fibrosis and applicable in routine clinical practice or as study outcome in a predominantly paediatric population [1– 6] . Other outcomes, such as the slope of phase III parameters (S acin and S cond ), have been found to predict disease course in asthmatic patients [7–10] and other respiratory conditions, but predominantly in adult populations [11–13] . An important outcome of this renaissance is the recently published ERS/ATS consensus statement on inert gas washout tests [14] . This document gives an overview on existing techniques, practical suggestions with regard to the measurement itself and clear advice for validating new equipment. Importantly, it highlights the need for further characterization and validation work with regard to the test procedure, equipment and analysis methods. This cannot be emphasized enough, as comparable validation studies were never done for basic tests that are Published online: April 24, 2014