BackgroundPulmonary manifestations are the major cause of morbidity and mortality in patients with inborn errors of immunity (IEI). New and more sensitive diagnostic methods can potentially lead to earlier recognition and treatment of IEI lung disease and improve outcome. The aim of this study was to compare multiple-breath washout (MBW) and spirometry in patients with IEI and cystic fibrosis (CF) as well as healthy controls (HC) and to evaluate the sensitivity of lung clearance index (LCI) to assess lung disease in IEI.MethodsIEI patients (n=114) were recruited from our paediatric and adult immunodeficiency outpatient clinics and compared to age-matched CF patients (n=114) and HC (n=114). MBW measurements and spirometry were performed in the study participants, and MBW testing was repeated after 63–707 days in IEI patients (n=70).ResultsThe LCI was significantly higher in IEI patients than in HC (p<0.001) and significantly lower than in CF patients (p<0.001). The forced expiratory volume in 1 s (FEV₁) z-score was significantly lower in IEI patients than in HC (p<0.01) and significantly higher than in CF patients (p<0.01). LCI and FEV₁z-score correlated moderately negatively in the total cohort, the IEI group and the CF group. Nineteen (20.7%) of 92 IEI patients and 35 (33.3%) of 105 CF patients had an elevated LCI but a normal FEV₁z-score. After a median of 364 days, the median LCI of 70 IEI patients increased significantly by 0.2.ConclusionMBW is useful to detect lung disease in IEI and is more sensitive than spirometry.