The Impact of Airway Obstruction on Feno Values in Asthma Patients.

Background: Exhaled nitric oxide (Feno) is used as a marker of type-2 airway inflammation in asthma management. Studies with airway challenges demonstrated that a reduction in airway caliber decreases Feno levels.

Objective: To evaluate the impact of airway caliber reduction occurring spontaneously in patients with asthma on Feno values in daily clinical practice.

Methods: In this post hoc analysis, Feno, FEV, and asthma control questionnaire scores were recorded on each visit for 120 (1073 visits) adult patients with asthma. Blood eosinophils were measured intermittently. The intraindividual relationship between Feno and FEV was evaluated via a linear mixed model. The determinants of the individual mean Feno were measured by a stepwise multivariate linear model including individual mean FEV, inhaled corticosteroid dose, asthma control questionnaire score, and blood eosinophils.

Results: Variations in the negative Feno-FEV relationship within individuals at different times were significantly determined by the individual's mean FEV. This relationship did not hold for individuals above the 75th and below the 25th quartiles. The best explanatory variables for individual mean Feno were FEV (+4.3 parts per billion/10%pred) and blood eosinophil count (+1 part per billion per 100 cells/mm).

Discussion: In the presence of variable degrees of heterogeneous patterns of airway inflammation, airway caliber is shown to be an independent and significant determinant of Feno when measured in patients with asthma. We would propose a +4-parts-per-billion correction factor to the measured Feno value for each 10% reduction below 100% predicted FEV. Doing this should improve the rigor of interpretation of Feno as an indicator of type-2 inflammation in patients with low FEV.