Sputum induction leads to a decrease of exhaled nitric oxide unrelated to airflow

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Abstract
Measurement of exhaled nitric oxide (eNO) and analysis of induced sputum are both established noninvasive methods for studying airway inflammation in asthma. Both methods are often used sequentially within short time frames. The aim of the present study was to evaluate the influence of sputum induction on eNO in adults and to follow the kinetics of airway eNO production after induction in relation to forced expiratory volume in one second (FEV1).eNO and FEV1were measured in 41 adult patients (aged 29 (range 23–50) yrs, 56% male) with asymptomatic atopy or mild asthma (mean FEV197.2±3% predicted) prior to and immediately after sputum induction with hypertonic saline (4%). Sputum induction with isotonic saline was also performed in 13 subjects (control group). Repeatability of eNO decrease after sputum induction was also studied in 27 patients on separate occasions and kinetics of eNO production after sputum induction were followed over 24 h in another 10 patients.Sputum induction with hypertonic, but not isotonic, saline led to a marked decrease in eNO (log) immediately after the procedure (pre: 3.85±0.13 parts per billion (ppb); post: 3.24±0.14 ppb). This decrease was shown to be highly reproducible and not related to a fall in FEV1following sputum induction. While FEV1returned to baseline within 1 h, decreased eNO levels were observed over 4 h and returned to baseline after 24 h.Hypertonic saline sputum induction leads to a prolonged reduction in exhaled nitric oxide in adult atopics that is reproducible within subjects and not related to a reduction in airflow following sputum induction. This methodological interference should be taken into account when sputum induction and exhaled nitric oxide measurements are performed in the same subject.