Background: in the clinical practice patients with suspicion of asthma but a normal spirometry are often sent to a Methacholine challenge test (MCT) to further investigation. Methods that assess ventilation heterogeneity through inert gas washout have showed high sensitivity in the diagnosis of other respiratory diseases and became recently standardized.

Aims and objectives: we hypothesized that nitrogen single or multiple breath washout tests, respectively N2SBW and N2MBW, may be pathological in patients with suspicion of asthma and normal spirometry and we wanted to compare them with MCT.

Methods: this prospective, observational, single-center study included patients with suspicion of asthma and normal spirometry. Participants underwent comprehensive lung function assessments including MCT, baseline N2MBW and N2SBW. Additionally, N2SBM was performed during MCT after every methacholine incremental administration. 

Results: 182 patients were screened and 106 were included in the study. The average age of the participants was 41.8±1.4 years, 42% were smokers and the majority were female (62%). MCT was abnormal in 48% of subjects, N2SBW was pathological in only 11% and Lung clearance index (LCI) from N2MBW in 81%. The dose response rate of the MCT showed weak to moderate correlation with the subsequent N2SBW measurements during provocation phases one to six (rho 0.34 ? 0.49). 

Conclusions: both MCT and N2MBW are frequently pathological in patients with suspicion of asthma and normal spirometry. The weak correlation and lack of concordance between the tests, however, merits further investigation to better understand the involved pathological pathway.