A method for the standardized offline collection of exhaled nitric oxide

Study objectives: Exhaled nitric oxide (ENO) is a noninvasive marker of air way ay inflammation:. The purpose of this study was to compare a standardiz ed offline ENO measurement :apparatus with a validated on-line method, Design: Asthmatic volunteers (n = 21) had ENO measured by the the two follo wing methods: (1) inhalation to total lung capacity (TLC) followed by, exha lation at a constant flow (45 mL/s) against a high resistance, while monito ring nitric oxide (NO) and pressure on-line; and (2) inhalation to TLC and exhalation into mylar balloons via an apparatus that included the same resi stance and flow rate as used in the on-line method. We also examined NO sta bility in mylar balloons over 48 h. Measurements and results: ENO values (given as geometric mean in parts pel billion [ppb]; 95% confidence intervals) differed between the on-line metho d (69.6; 12.6 to 113.8) and the offline method(49.5; 30.9 to 79.3), indicat ing 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), h owever, showed excellent correlation between the on-line and offline method s. Within-subject repeatability, as assessed 1,by the coefficient of repeat ability (CR), was good for both the on-line and. offline methods (CK, 1.09 and 1.17, respectively). Geometric mean NO concentrations (95% confidence l imits) in mylar balloons containing exhalate increased from a baseline of 5 5.8 ppb (30.9, 84.4) to 64.5 ppb (45.6, 91.1) and 69.5 ppb (51.4, 94.0) at 24 h and 48 h, respectively . Conclusions: The offline method gave reproducible ENO values that were cons istently smaller than, but showed good cor 1 elation with, values obtained with:ith on-line ENO collection. This method is suitable for offline collec tion, but the measured values are not interchangeable with those obtained b y on-line measurement.