D1.103 - Two cases of discrepancy between fractional exhaled nitric oxide and airway eosinophils in asthma

Background

It is known that fractional exhaled nitric oxide (FeNO) may reflect the degree of airway eosinophilic inflammation in asthma. In practice, FeNO is used to indirectly assess airway eosinophilia and monitor the response to inhaled corticosteroid (ICS). However, we describe two asthma patients who showed a discrepancy between FeNO levels and sputum eosinophil counts.

Methods

FeNO levels were measured periodically to monitor the response to ICS. Meanwhile, induced sputum test was performed to directly assess airway eosinophilic inflammation.

Results

A first case of woman was diagnosed with house dust mites-induced allergic asthma, based on methacholine bronchial provocation test and skin prick test. Forced expiratory volume in one second (FEV1) was 2.6 L (86% of predicted). Blood eosinophils were 1,100/μL and eosinophilic cationic protein (ECP) was 77.2 μg/L. FeNO was 144 ppb (reference range < 25). She has been treated regularly with a combination of ICS and long acting beta2 agonist (LABA), with and without montelukast and/or theophylline from May 2016. Thereafter, FeNO has been measured 9 times more to monitor the response of asthma medications. However, FeNO was not reduced, although asthma has been controlled, ranging from 114 ppb to 192 ppb. At the time of the highest FeNO measurement, induced sputum test was performed, showing 0% eosinophils, indicating that elevated FeNO levels persisted independently of airway eosinophilic inflammation. A second case of man was diagnosed with nonallergic asthma, based on bronchodilator reversibility test and skin prick test. FEV1 was 2.5 L (61% of predicted). Blood eosinophils were 300/μL and ECP was 13.1 μg/L. His asthma has been controlled with the combination of ICS and LABA, with and without montelukast and/or theophylline from July 2012. FeNO was 99 ppb, which was measured at October 2014. Thereafter, FeNO has been measured 10 times more. However, FeNO was not reduced to normal range, although asthma has been controlled, ranging from 68 ppb to 173 ppb. Induced sputum test showed 2.3% eosinophils, suggesting that elevated FeNO was not associated with airway eosinophilic inflammation.

Conclusions

The two cases illustrate that FeNO can remain elevated even in the absence of eosinophilic airway inflammation in patients with controlled asthma. This suggests that under certain circumstances, FeNO may not accurately reflect airway eosinophilic inflammation, particularly when monitoring the response to ICS in asthma.