Fractional exhaled nitric oxide (FeNO) is convenient to use as a biomarker of airway inflammation. However, the mutual relationship between FeNO, peripheral blood eosinophil, total immunoglobulin E (IgE), and inflammatory cytokines showed some controversy. This study was carried out to determine the accuracy of peripheral blood eosinophil and total IgE to detect eosinophilic airway inflammation as determined by two FeNO cutoff points. The correlation between FeNO, peripheral blood eosinophil, total IgE, and specific inflammatory cytokines was also examined.
Seventy-six patients with partly controlled asthma performed the following tests on the same day: FeNO, pulmonary function tests (PFTs), peripheral blood eosinophils, total IgE, and inflammatory cytokine assay. The correlation between these markers was investigated, and the diagnostic accuracy of peripheral blood eosinophils and total IgE to identify eosinophilic asthma phenotype was calculated using receiver operating characteristics area under the curve (ROC AUC). FeNO was positively correlated with percentage of blood eosinophils (r=0.276, p=0.017) and total blood IgE (r=0.3647; p=0.0013). No relationship between FeNO and serum inflammatory cytokines was detected. AUC of blood eosinophils and total IgE were 57% and 64% at FeNO ≥ 25 ppb and were 67% and 64% at FeNO > 50, respectively. The higher predictive ability was detected at FeNO > 50 ppb, where the best cutoff point for blood eosinophil % was ≥ 4.0% (sensitivity 66.7%, specificity 60.0%) and the best cutoff point for total IgE was ≥ 350 (sensitivity 66.7%, specificity 63.6%).
In patients with partly controlled asthma, peripheral blood eosinophil and total IgE showed equal useful accuracy in predicting eosinophilic airways. However, higher predictive values were reported at FeNO level > 50 ppb. FeNO was positively correlated with peripheral blood eosinophil, total IgE but not with any studied cytokines.