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The following is a summary of “Impact of Airway Obstruction on Feno Values in Asthma Patients,” published in the January 2024  issue of Allergy & Immunology by Michils,  et al.

In the treatment of asthma, exhaled nitric oxide (FeNO) is used to show that type 2 airways are inflamed. Studies with difficult airways showed that narrowing the airways lowers FeNO levels. 

For a study, researchers sought to find out what happens to FeNO values when people with asthma naturally have smaller airways in daily clinical practice. 

The post hoc study looked at results on the FeNO, FEV₁, and asthma control questionnaires at each visit for 120 adult asthma patients, adding up to 1,073 visits. Blood eosinophils were tested on and off. A linear mixed model examined the link between FeNO and FEV₁ within a single person. They used a stepwise multivariate linear model to determine what factors affect a person’s mean FeNO. These factors included the person’s mean FEV₁, the amount of inhaled corticosteroids, their asthma control questionnaire score, and the number of eosinophils in their blood. 

Differences in the negative FeNO-FEV1 link between people at different times were largely determined by their mean FEV₁ . People above the 75th and below the 25th quartiles did not have this connection. For each person, FEV₁ (+4.3 parts per billion/10%pred) and blood eosinophil count (+1.1 parts per billion per 100 cells/mm ³) were the best predictors of mean FeNO. When measuring FeNO in asthma patients, airway diameter is shown to be an independent and significant factor. This is true even when there are different levels of inflammation in the airways. They suggest adding +4 parts per billion to the observed FeNO number for every 10% drop below the 100% expected FEV₁. This step should make it easier to read FeNO as a sign of type-2 inflammation in people with low FEV₁.

Source: sciencedirect.com/science/article/abs/pii/S2213219823009492