Performance of two portable exhaled nitric oxide fraction devices compared to a "gold standard" chemiluminescence device

Background: Recent guidelines for the diagnosis of asthma in schoolchildren recommend the measurement of exhaled nitric oxide fraction (F$_{ENO}$) as part of the diagnostic algorithm. However, implementation may be hampered by the lack of F$_{ENO}$ devices that are affordable and usable in all healthcare settings. We aimed to compare the performance of two portable F$_{ENO}$ devices (Evernoa (EVE) and NObreath (NOB)) to a stationary "gold standard" device (CLD 88 sp (CLD)) in children. Methods: 106 children aged 6-17 years under investigation or monitoring for asthma underwent F$_{ENO}$ measurements using the three devices in randomised order. Results: All devices showed high repeatability across a wide F$_{ENO}$ range (2.5-191.9 ppb). Median (interquartile range) F ENO levels were significantly lower with the portable devices (20.8 (9.5-43.5), 16 (9-36) and 22.8 (13.2-55.2) ppb for NOB, EVE and CLD, respectively; p<0.0001). Despite the proportional bias (-20% (NOB) and -40% (EVE)), both portable devices demonstrated good overall agreement with CLD (>94%) at a cut-off level of 25 ppb but lower agreement for a cut-off of 35 ppb. EVE required a greater number of attempts compared to NOB and CLD to achieve two valid measurements. Conclusions: Both portable devices showed limited interchangeability with the gold standard, making them less applicable for research and disease monitoring purposes. However, good overall agreement at the European Respiratory Society cut-off level (25 ppb) suggests potential as a simple and convenient screening tool in clinical settings for initial asthma diagnosis.