The quality of evidence is downgraded by study limitations (lack of blinding).
A Cochrane review [Abstract] 1 included 7 studies with a total of 1 700 adult subjects (1 546 completed) with asthma. The studies differed in definition of asthma exacerbations, fractional exhaled nitric oxide (FeNO) cutoff levels used (15 to 35 ppb), the way in which FeNO was used to adjust therapy, and duration of study (4 to 12 months). The mean ages of the participants ranged from 28 to 54 years old.
The number of people having one or more asthma exacerbations was significantly lower in the FeNO group compared to the control group (OR 0.60, 95% CI 0.43 to 0.84; 5 studies, n=1 005). The number needed to treat to benefit (NNTB) over 52 weeks was 12 (95% CI 8 to 32). Those in the FeNO group were also significantly more likely to have a lower exacerbation rate than the controls (rate ratio 0.59, 95% CI 0.45 to 0.77; 5 studies, n=842). However, no difference was found for exacerbations requiring hospitalisation (OR 0.14, 95% CI 0.01 to 2.67; 3 studies, n=488) or rescue oral corticosteroids (OR 0.86, 95% CI 0.50 to 1.48; 3 studies, n=495). There was also no significant difference between groups for any of the secondary outcomes (FEV1, FeNO levels, symptoms scores, or inhaled corticosteroid doses at final visit).
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