Evaluation of the peripheral blood eosinophil count as a predictor for fractional exhaled nitric oxide or bronchodilator reversibility test outcome.

Objective: This study aimed to explore the usefulness of the peripheral blood eosinophil count (PBEC) in assessing the level of fractional exhaled nitric oxide (FeNO) and predicting bronchodilation test results. Methods: We retrospectively analyzed the data of 384 outpatients who underwent FeNO measurement at our Department of Respiratory and Critical Care Medicine from March to June 2019. The FeNO level was compared among different PBECs to explore the association among them. Furthermore, the sensitivity and specificity of PBECs in predicting bronchodilation test results were assessed by using receiver operating characteristic (ROC) curve analysis. Results: There was a moderate correlation between PBECs and FeNO levels (r = 0.414; p < 0.05). In the subjects with PBECs ≥ 0.3 × 109/L, the median FeNO level was 39 ppb (interquartile range, 22.5-65.5 ppb), significantly higher than in the subjects with PBECs < 0.3 × 109/L. The area under the ROC curve was 0.707 (p < 0.05). The maximum Youden index (0.348) was at PBECs = 0.205 × 109/L, which achieved sensitivity and specificity of 63% and 71.8%, respectively. Conclusion: PBECs ≥ 0.3 × 109/L can predict a positive bronchodilation test result and a high FeNO level, with a probability of 50% in the subjects with chronic cough and shortness of breath; in the absence of corresponding symptoms and a low PBEC, the predictive value was small. For hospitals not able to conduct FeNO measurements, for outpatients with poor economic conditions, and for patients with confirmed or suspected novel coronavirus disease 2019, the PBEC, in conjunction with a patient's clinical symptoms, can improve the diagnostic accuracy of allergic asthma and assessment of airway inflammation while reducing the risk of infection.