Cutoff point for exhaled nitric oxide corresponding to 3% sputum eosinophils

The eosinophilic asthma phenotype (sputum eosinophils 3%) indicates a good response to corticosteroids and T(H)2 immunomodulators. Exhaled nitric oxide (FeNO) is rapidly measured by portable devices, and although it is not a selective marker of eosinophilic inflammation, several studies have demonstrated a strong correlation with it. We investigated which FeNO value was the best fit with 3% sputum eosinophils in asthma patients.

We included 129 consecutive, nonsmoking asthmatics who underwent skin tests, FeNO quantification (NIOX MINO), spirometry, and induced sputum analysis and completed the Asthma Control Test questionnaire. Receiver operating characteristic curves were constructed, and logistic regression analysis was performed.

Symptoms were detected more frequently in the eosinophilic asthma group, as were higher airway obstruction and sensitivity to hypertonic saline. The FeNO cutoff point of 21 ppb was the best fit with 3% sputum eosinophilia. This value behaved better among corticosteroid-naïve patients (sensitivity, 97%; specificity, 58%; positive predictive value, 86%; negative predictive value, 88%) than among those receiving corticosteroids (sensitivity, 81%; specificity, 25%; positive predictive value, 74%; negative predictive value, 33%).

FeNO ≥ 21 ppb is associated with airway eosinophilia. In corticosteroid-naïve patients, FeNO < 21 ppb enables us to rule out airway eosinophilia.