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Malmberg and colleagues reported the robust discriminatory properties of exhaled nitric oxide (FENO) for asthma in a paediatric population, but also noted that 29% of the subjects studied could not perform the manoeuvres necessary for online NO measurements at a target expiratory flow rate of 50 ml/s.1 These results are consistent with those reported by Canady and colleagues who noted that 24% of children studied could not perform online NO analysis.2 We studied healthy and asthmatic adults and found a similarly robust ability of NO to discriminate between those with and without asthma with online technique and flow rate of 50 ml/s (area under ROC curve 0.84).3 Importantly, we also found that these discriminatory properties were not diminished when simpler offline collection techniques or faster, more tolerable, expiratory flow rates were used (areas under ROC curve 0.79–0.86). If NO measurements are to gain acceptance for identifying children with asthma, use of offline techniques with faster expiratory flow rates may be preferred.
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Author’s reply
We appreciate Dr Deykin’s comments regarding our recent study comparing the diagnostic power of exhaled nitric oxide (FENO) and the oscillometric assessment of lung function for asthma in preschool children.1 We are pleased to learn that our results of the robust discriminatory properties of FENO are consistent with his findings in healthy and asthmatic adults.2 Importantly, Dr Deykin also mentions the feasibility problems of standard online FENO measurements in young children3 which we and others have found in a series of preschool children. As discussed in our paper,1 the standard online technique requires considerable cooperation and, according to our experience, is rarely successful in children aged less than 4 years.
Dr Deykin’s proposal of using offline measurements has practical advantages over the standard technique which relate to the portability of the samples. However, because of the flow dependence of FENO, standardisation of the flow rate is necessary even when using this technique, so the measurement may not be significantly easier for the child than the online method. In commercial equipment, dynamic resistors and biofeedback views on the computer screen may increase the feasibility of online measurements in young children, but there is still a need to develop new techniques and recommendations for the measurement of FENO in children of preschool age and in infants.3 The findings of Dr Deykin and colleagues that offline measurements, when controlled at low and faster flow rates, maintain good discriminatory properties for asthma are certainly important when such recommendations are to be updated. However, further studies are necessary to see whether these results can be extrapolated and applied to young children.