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Background  Currently, there is much interest in measuring fractional exhaled nitric oxide ( FE NO  ) in populations. We evaluated the reproducibility of  FE NO   in healthy subjects and determined the number of subjects necessary to carry out a longitudinal survey of  FE NO   in a population containing smokers and non-smokers, based on the assessed reproducibility.    Methods  The reproducibility of  FE NO   was examined in 18 healthy smokers and 21 non-smokers.  FE NO   was assessed once at 9 AM on five consecutive days; in the last day this measurement was repeated at 2 PM. Respiratory symptoms and medical history were assessed by questionnaire. The within- and between-session repeatability of  FE NO   and log-transformed  FE NO   was described. The power of a longitudinal study based on a relative increase in  FE NO   was estimated using a bilateral t-test of the log-transformed  FE NO   using the between-session variance of the assay.    Results   FE NO   measurements were highly reproducible throughout the study.  FE NO   was significantly higher in males than females regardless of smoking status.  FE NO   was positively associated with height (p < 0.001), gender (p < 0.034), smoking (p < 0.0001) and percent FEV 1 /FVC (p < 0.001) but not with age (p = 0.987). The between-session standard deviation was roughly constant on the log scale. Assuming the between-session standard deviation is equal to its longitudinal equivalent, either 111 or 29 subjects would be necessary to achieve an 80% power in detecting a 3% or a 10% increase in  FE NO   respectively.    Conclusion  The good reproducibility of  FE NO   is not influenced by gender or smoking habits. In a well controlled, longitudinal study it should allow detecting even small increases in  FE NO   with a reasonable population size.

Reproducibility of exhaled nitric oxide in smokers and non-smokers: relevance for longitudinal studies