Repeatability of successive measurements with a portable nitric oxide analyser in patients with suggested or diagnosed asthma.

The objective of our study was to determine the repeatability of successive measurements of exhaled nitric oxide (NO) with a portable device (NIOX MINO). We assembled a sample of 332 pairs of repeated NO measurements acquired in the same session. The mean of the two successive measurements varied from 4.0 to 279.5 parts per billion (ppb). The mean coefficient of repeatability (CoR) of the entire study sample was 2.4 ppb. The difference between paired measurements increased slightly with increasing fractional exhaled nitric oxide (FENO); for the mean FENO of 24.0 ppb the calculated CoR was 2.2 and for the FENO of 54 ppb it was 3.2 ppb. The difference between paired observations for FENO values <50 ppb was significantly lower than that for values >50 ppb (p<0.001). The demonstrated repeatability of successive measurements with the device supports the view that, when the exhalation manoeuvre is technically valid and FENO is within normal limits, repeated determinations of FENO are generally not necessary for screening. In other cases, the mean value of two successive measurements would represent a more reliable measurement result.