BACKGROUND The aim of this study was to validate exhaled nitric oxide (eNO) values obtained with an alternative off line, single breath, low flow balloon sampling method against on line sampling according to ERS and ATS guidelines in children who could perform both methods.

RESULTS The off line eNO values were significantly higher than the on line values at all flows. At 50 ml/s the geometric mean (SE) off line eNO was 18.7 (1.1) ppb and the on line eNO was 15.1 (1.1) ppb (p<0.0001). However, when dead space air was discarded, off line and on line values were similar: at 50 ml/s off line eNO was 17.7 (1.0) ppb and on line eNO 16.0 (1.2) ppb. There was a good agreement between off line eNO values without dead space air and on line eNO: for 50 ml/s the mean on/off line ratio was 0.95 (95% agreement limits 0.63 to 1.27). The off line eNO level at 50 ml/s in 80 children with negative questionnaires for asthma, rhinitis, and eczema was 13.6 (1.0) ppb compared with 33.3 (1.1) ppb in the remaining children with positive questionnaires on asthma and allergy and/or recent symptoms of cold (p<0.0001).

CONCLUSIONS In children, off line assessment of eNO using constant low flow sampling and excluding dead space air is feasible and produces similar results as on line assessment with the same exhalation flow rate. Both sampling methods are sufficiently sensitive to differentiate between groups of otherwise healthy school children with and without self-reported asthma, allergy, and/or colds. We propose that, for off line sampling, similar low flow rates should be used as are recommended for on line measurements.