Study objectives: Exhaled nitric oxide (ENO) is a
noninvasive marker of airway inflammation. The purpose of this study
was to compare a standardized offline ENO measurement apparatus with a
validated on-line method.
Design: Asthmatic volunteers
(n = 21) had ENO measured by the two following methods: (1)
inhalation to total lung capacity (TLC) followed by exhalation at a
constant flow (45 mL/s) against a high resistance, while monitoring
nitric oxide (NO) and pressure on-line; and (2) inhalation to TLC and
exhalation into mylar balloons via an apparatus that included the same
resistance and flow rate as used in the on-line method. We also
examined NO stability in mylar balloons over 48 h.
Measurements and results: ENO values (given as geometric
mean in parts per billion [ppb]; 95% confidence intervals) differed
between the on-line method (69.6; 42.6 to 113.8) and the offline method
(49.5; 30.9 to 79.3), indicating that the offline method gave lower ENO
measures than the on-line method (p < 0.001). Furthermore,
this difference between measures increased with increasing mean values.
The intraclass correlation coefficient (0.931), however, showed
excellent correlation between the on-line and offline methods.
Within-subject repeatability, as assessed by the coefficient of
repeatability (CR), was good for both the on-line and offline methods
(CR, 1.09 and 1.17, respectively). Geometric mean NO concentrations
(95% confidence limits) in mylar balloons containing exhalate
increased from a baseline of 55.8 ppb (36.9, 84.4) to 64.5 ppb (45.6,
91.1) and 69.5 ppb (51.4, 94.0) at 24 h and 48 h,
Conclusions: The offline method
gave reproducible ENO values that were consistently smaller than, but
showed good correlation with, values obtained with on-line ENO
collection. This method is suitable for offline collection, but
the measured values are not interchangeable with those obtained by