The Long-Term Variability of FeNO in Pregnant Asthmatic Women with Controlled Asthma

Introduction: Fractional exhaled nitric oxide (FeNO) is considered a useful, noninvasive marker of airway inflammation in asthma and allergic rhinitis. It has also been suggested that anti-inflammatory treatment guided by monitoring of exhaled NO could improve overall asthma control. However, long-term intrasubject variability of this parameter as well as the rate of its change, which can be considered clinically significant, have not been established yet. The aim of our study was to assess the long-term variability of FeNO in pregnant asthmatic women with controlled asthma. Material and methods: Pregnant, non-smoking women with asthma were recruited at between 3 and 6 months of gestation. Exhaled nitric oxide (FeNO) spirometric parameters were measured, and asthma control tests (ACT) were completed at monthly visits up to delivery. The data of 26 subjects with well controlled asthma during pregnancy (ACT values within the range 20–25, normal spirometric parameters, stable treatment) were analysed. The variability of FeNO values was assessed using the variation coefficient CV (standard deviation × 100%/arithmetic mean). Results: The median level of FeNO coefficient of variation (CV) was: 33.8% (range 11.3 to 121.9) in all subjects with well controlled asthma during pregnancy. There were no statistically significant differences in FeNO variability between groups of patients who had at least one measurement of FeNO higher than 50 ppb (39%; 11.8–121.9%) and those with all FeNO values below 50 ppb (29.9%; 11.3–71.8%), as well as between atopic (35.7%; 11.8–121.9%) and nonatopic (24.2%; 11.3–71.8%) pregnant asthmatics (p = 0.95 and 0.11, respectively). Conclusions: High long-term variability of fractional exhaled nitric oxide values revealed in pregnant women with well controlled asthma indicates that changes in this parameter should be interpreted with caution when used for asthma treatment monitoring.