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Background and Objective: In patients with asthma, fractional exhaled nitric oxide (FeNO) is a biomarker for eosinophilic airway inflammation. However, the correlation with the blood eosinophil count, immunoglobulin E (IgE), and spirometric values is not well established. We aimed to investigate the correlation between FeNO, blood eosinophils, IgE, and spirometric values in asthmatic patients., Methods: Data were extracted from the electronic medical records of adult asthmatic patients, and included the FeNO, blood eosinophil count, IgE, and spirometric values. The correlation between the markers was investigated using receiver operating characteristics (ROC) and the area under the curve (AUC)., Results: A total of 135 adult patients (30% male) were analyzed. The mean FeNO was 36 ppb and almost half (48%) had a FeNO > 25 ppb. The mean blood eosinophil count was 377 cells/mm 3 , and 42% had a blood eosinophil count > 300 cells/mm 3 . There was a significant positive correlation between FeNO and the blood eosinophil count (r = 0.42, sensitivity 63%, specificity 77%, AUC 72%, p < 0.0001) as well as with the IgE (r = 0.35, sensitivity 78%, specificity 44%, AUC 68%, p < 0.0007). There was significant negative correlation between FeNO and FEV1% (r = -0.18, sensitivity 35%, specificity 85%, AUC 67%, p = 0.027)., Conclusion: The blood eosinophil count, IgE, and spirometric values were correlated with the severity of the eosinophilic airway inflammation (high FeNO). Asthmatic patients with a higher FeNO level had a higher eosinophil count, higher IgE levels, and lower FEV1 values., Competing Interests: The authors have declared that no competing interests exist., (Copyright © 2023, Al Ghobain et al.)