Long-term reproducibility of exhaled nitric oxide and exhaled breath temperature in healthy children.

Background Measurements of the fractional concentration of exhaled nitric oxide (FENO) and exhaled breath temperature (EBT) are easy to perform, non-invasive and considered reproducible at short-term; data on their long term reproducibility is needed. Aims To assess reproducibility of FENO and EBT in healthy children one-year apart and their relationship with lung function and anthropometric changes. Methods In 41 healthy children (24 males) aged 9.8±1.6 yr, range 8.0-13.8, we obtained duplicated measurements of FENO and EBT in two sessions one-year apart (2011 and 2012). Subjects did also spirometry in both sessions. The intraclass coefficient correlation (ICC) and the coefficient of repeatability (CR=2 SD) were assessed. Overtime changes in variables (“D” prefix) were calculated. Results Intra-session reproducibility of duplicate measurements was high in both sessions [(ICC >0.94 for both FENO and EBT); (CR: FENO 5.2 ppb and 3.0 ppb, EBT 1.8 and 1.2 °C)]. Geometric means (95% confidence interval) did not change significantly from the 2011 to the 2012 sessions [FENO: 10.6 (9.3-12.2) ppb and 10.2 (9.2-11.3) ppb; EBT: 32 (31.3-32.7) °C and 32.6 (31.9-33.2) °C]. Reproducibility decreased after 1 year [FENO (ICC: 0.48, CR: 11.0 ppb); EBT (ICC: 0.75, CR: 3.5 °C)]. The between-year percent increase in FENO correlated with decreased lung function (DFEV1% r=-0.33, DFEF25-75 % r=-0.39, p<0.05) whereas EBT increased with BMI (DBMI 0.41, p=0.008). Conclusion Reproducibility of both FENO and EBT decreases after one year in healthy children; their changes have a different meaning; FENO is more affected by changes in bronchial patency whereas EBT by increases in body mass index.