Objective: Guidelines suggest using bronchial provocation testing (BPT), which is hard to attain, in patients with asthma-like symptoms presenting with nondiagnostic spirometric tests. To eliminate the risk of over/underdiagnosing asthma, we aimed to evaluate the predictive value of not only fractional exhaled nitric oxide (FeNO) but also other easily accessible clinical indices for ruling in/out asthma.

Materials and Methods: This retrospective study included adults presenting to our clinic with respiratory symptoms suggestive of asthma but with normal spirometric values and negative reversibility test, who underwent FeNO and methacholine BPT (MchBPT). Medical records were used to obtain descriptive characteristics, clinical history, allergy screening, eosinophils in peripheral blood, and spirometry.

Results: Among 51 patients, 19 were diagnosed with asthma. Body mass index and blood eosinophils were significantly higher in patients with positive MchBPT (p=0.042 and p=0.037, respectively). No significant difference was found in other indices, including FeNO (p=0.293). Receiver operating characteristic curve analysis revealed the best diagnostic cutoff level for FeNO as 14 ppb and blood eosinophil as 150/μl for the prediction of positive MchBPT (with 63.16%-62.5% and 80%-61% sensitivity-specificity, respectively). These two indices were the only independent predictors of positive BHR, and the model of FeNO>14ppb combined with eos>150/μl showed 100% specificity with a 100% negative predictive value.

Conclusion: Our results suggest using the combination of FeNO with blood eosinophil count as a rule-out test, adding a new step in the algorithmic diagnosis of asthma. This might avoid an unnecessary BPT procedure, reduce the risk of over/under-diagnosis of asthma, and hasten the correct diagnosis.