Exhaled nitric oxide: sources of error in offline measurement

Abstract

Delayed offline measurement of exhaled nitric oxide (eNO), although useful in environmental and clinical research, is limited by the instability of stored breath samples. The authors characterized sources of instability with the goal of minimizing them. Breath and other air samples were stored under various conditions, and NO levels were measured repeatedly over 1-7 d. Concentration change rates varied positively with temperature and negatively with initial NO level, thus "stable" levels reflected a balance of NO-adding and NO-removing processes. Storage under refrigeration for a standardized period of time can optimize offline eNO measurement, although samples at room temperature are effectively stable for several hours.

Fig. 1

Nitric oxide (NO) concentration changes with time in samples from 3 healthy (A, B, C) and 1 mildly asthmatic (D) adult subjects in 10 different used Sievers bags (numbered 1–10) stored at 22 °C exposed to ambient NO.

Fig. 2

Rates of change in nitric oxide (NO) concentration at 4 storage temperatures, for all samples initially ≤ 20 ppb. Point = mean; flag = standard error.

Fig. 3

Rates of change in nitric oxide (NO) concentration as a function of initial NO concentration, for all samples stored at 6 °C. Lines indicate best-fit quadratic regressions for breath and other air samples. Difference between breath and other samples was not statistically significant.