Fractioned exhaled nitric oxide (FENO) is not a sufficiently reliable test for monitoring asthma in pregnancy

FENO measured at the beginning of the study did not show significant correlation with retrospectively evaluated asthma severity (r = 0.07; p = 0.97). An analysis of data collected during all 254 visits showed that FENO correlated significantly but weakly with ACT scores (r = 0.25; p = 0.0004) and FEV1 (r = 0.21; p = 0.0014). FENO at consecutive visits in women with well-controlled asthma (N = 50) showed large variability expressed by median coefficient of variation (CV) = 32.0% (Min 2.4%, Max 121.9%). This concerned both: atopic and nonatopic groups (35.5%; and 26.7%, respectively). Large FENO variability (35.5%) was also found in a subgroup of women (N = 11) with ACT = 25 constantly throughout the study. FENO measured at visits when women temporarily lost control of asthma (N = 22; 38 visits), showed an increasing tendency (64.2 ppb; 9.5 ppb-188.3 ppb), but did not differ significantly (p = 0.13) from measurements taken at visits during which asthma was well-controlled (27.6 ppb; 6.2 ppb-103.4 ppb). The comparison of FENO in consecutive months of pregnancy in women who had well-controlled asthma did not show significant differences in FENO values during the time of observation. The assessment of asthma during pregnancy by means of monitoring FENO is of limited practical value due to this parameter's considerable intrasubject variability, regardless of the degree of asthma control.