Exhaled NO has been studied extensively over the past decade and reports of the clinical utility of this method in the management of patients with asthma are now appearing in the literature [8, 9]. However, the introduction of the method into clinical routine has been restricted by the cost and complexity of existing NO analysers. In this study, the performance of a new hand-held device for exhaled NO measurements has been compared with that of a standard stationary unit.
When we compare the mean of three valid FENO measurements using the established chemiluminescence-based NIOX and the NIOX MINO, which incorporates an electrochemical sensor, the results suggest clinically acceptable agreement between the two instruments. Measured FENO levels obtained using the NIOX MINO were on average slightly higher than those obtained with the NIOX, and there was a tendency that the intrasubject FENO difference increased with increasing FENO. We believe that the difference between the two instruments is acceptable, considering the different measurement technologies and calibration procedures used in the two devices, and the results are in conformity with the declared accuracy for both the NIOX and the NIOX MINO. From a clinical point of view, accuracy will be more important in a FENO range close to a cut-off between health and disease (20–35 ppb) than at higher FENO levels. The NIOX MINO showed good agreement (within 95% limits of agreement) with the NIOX up to approximately 60 ppb, which indicates that the new hand-held device will be able to give clinical guidance with similar accuracy as the conventional chemiluminescence-based unit.
In general, the NIOX MINO and the NIOX had similar repeatability, except for one extreme observation with poor reproducibility in the NIOX MINO. However, this was seen in a subject with very high exhaled NO values (range 125–147 ppb in the NIOX MINO), and such variability at these high NO levels is of minor clinical importance. The repeatability agreed with the devices' technical specifications.
Success rates in achieving the required number of acceptable measurements were at least 84% for both devices and for both subject groups. Since all subjects were considered unexperienced with NO measurements, this indicates that both measurement techniques are generally well accepted by the patients. However, younger children failed slightly more frequently than adults when attempting to use the NIOX MINO. Interestingly, the number of attempts needed for successful subjects to achieve three acceptable measurements was significantly lower in the NIOX MINO compared to the NIOX. This could at least partly be explained by the fact that some measurements in the NIOX may be discarded after a linear regression analysis of the NO plateau has been performed, even though the number of regression failures was not recorded in the present study. The linearized plateau must not deviate more than 10% from the horizontal axis according to current guidelines [13]. In the NIOX MINO, the NO level in the last 3-s portion of mixed exhaled air is analyzed. Thus, the need for an analysis of the quality of a real-time NO plateau is avoided in the hand-held instrument.
Four subjects were excluded because of a low exhaled NO value (< 8 ppb). However, three of these subjects had a measurement above 5 ppb which is now the established lower detection limit of the NIOX MINO.
During the simulated home use, subjects were given the opportunity to use the NIOX MINO unassisted by study staff (children were assisted by their parents as they likely would be at home). This was performed after the clinical session, which would imitate what would normally happen, namely that the patient would receive training in the clinic prior to taking home the device. All subjects that succeeded in the clinical setting also succeeded in the simulated home environment. Using the mean of three measurements was advised in earlier guidelines [16], but this was recently changed to two measurements [13]. We found essentially the same agreement between the two devices when comparing the mean of three valid measurements in the NIOX and the first valid measurement in the NIOX MINO. We thus suggest that one measurement is adequate when using the NIOX MINO, which would save valuable time in the clinic. The time for NO analysis in the NIOX MINO is 100 s, but since one measurement seems to be adequate in most instances, the total measurement time will still be acceptable.