Diagnosing Asthma via Fractional Exhaled Nitric Oxide and Blood Eosinophil Count

The presence of eosinophils in these latter organs.
The presence of eosinophils in these latter organs.
Can fractional exhaled nitric oxide and blood eosinophil count serve as biomarkers for asthma when testing is not feasible?

Although neither fractional exhaled nitric oxide (FeNO) nor blood eosinophil (B-Eos) count alone is insufficient to accurately diagnose asthma, the combination of these 2 moderately elevated biomarkers may be sufficient for diagnosis, according to a retrospective cohort study published in BMC Pulmonary Medicine.

Finding a simple and effective method for diagnosing asthma is an urgent clinical problem because the common symptoms of asthma are relatively nonspecific. In addition, the recommended objective tests require complex cooperation from patients and long durations of examination time, and these tests may pose certain risks.

While FeNO and B-Eos count have been suggested as biomarkers to distinguish airway inflammation in asthma, previous studies have indicated clinicians may not rely upon either biomarker alone for an accurate asthma diagnosis. Therefore, researchers in China studied the diagnostic accuracy of using these 2 biomarkers in combination for the diagnosis of asthma.

Investigators examined 7463 suspected asthma cases between January 2014 and December 2019, identifying 2349 patients with complete FeNO, B-Eos count, and spirometry data. The diagnostic accuracy of FeNO or B-Eos alone or both in combination for asthma was then evaluated. Of the patients studied, 824 were diagnosed with asthma by clinicians (with respiratory symptoms and positive bronchial provocation or bronodilation test as criteria).

Data analysis found a weak correlation between FeNO levels and B-Eos counts in study participants, although the correlation was stronger in patients with asthma than in those without asthma. The analysis also found that neither biomarker alone was predictive of the asthma diagnosis. However, the 2 biomarkers used in combination accurately diagnosed asthma in 39.7% of the asthma patients studied. The misdiagnosis rate when using both biomarkers was 4.5%. 

Further analysis of combinations of different threshold values indicated that moderately elevated biomarker levels (FeNO >40 ppb and B-Eos >300 cells/μl) supported a diagnosis of asthma; diagnostic specificity was more than 95% and the positive likelihood ratio was more than 10.

The authors concluded, “There was no difference in diagnostic accuracy for asthma between FeNO and B-Eos count, and the combination of these two biomarkers could slightly improve diagnostic efficacy.” They added, “Patients with moderately elevated biomarkers (FeNO >40 ppb and B-Eos >300 cells/μl) could be diagnosed with asthma and avoid objective tests when such tests are not feasible.”


Li JH, Han R, Wang Y-B, et al. Diagnostic possibility of the combination of exhaled nitric oxide and blood eosinophil count for eosinophilic asthma. BMC Pulm Med. 2021;21(1):259. doi:10.1186/s12890-021-01626-z