Can FeNO Be Used To Diagnose Asthma?

A Simple Diagnostic and Monitoring Tool For Your Asthma

Shortness of Breath
Shortness of Breath. Getty Images E

Can fractional excretion of exhaled nitric oxide or FeNO be used to diagnose asthma?

The diagnosis of asthma can be difficult and there is no single test that can be used for a diagnosis.

While a large percentage of asthma is due to eosinophilic inflammation, patients whose asthma symptoms are due to other causes of inflammation (e.g. basophils, lymphocytes, or neutrophils) are not likely to benefit from FeNO testing.

According to a Clinical Practice Guideline from the American Thoracic Society, FeNO can be used to make a diagnosis of eosinophilic inflammation and support a diagnosis of asthma if objective evidence is needed.

More specifically, elevated levels of FeNO indicate inflammation in the lungs and that a patient is likely to respond to inhaled steroids based on previous research. Thus, FeNO more accurately identifies a potential response to treatment rather than making a specific diagnosis of asthma.

Problems With Traditional Monitoring

Most asthma action plans are currently driven by asthma symptoms or peak flows. Traditional symptoms such as wheezing, chest tightness, cough and shortness of breath can be very non-specific and are not necessarily related to the amount of inflammation that is actually present in your lungs. Peak flow may only be mildly correlated with the degree of airway inflammation according to some research.

Advantages of FeNO For Monitoring Asthma

About half of all asthma cases involve eosinophilic airway inflammation. This type of inflammation results from the upregulation of mast cells and other cells that specifically increase nitric oxide levels.

As previously stated, patients with eosinophilic inflammation have elevated FeNO levels, while patients with inflammation from other causes do not have elevated FeNO levels.

Additionally, studies have demonstrated biopsy proven inflammation in patients with elevated FeNO levels. As a result, elevated FeNO levels are thought to identify patients who are likely to respond to steroid treatment and studies have shown that elevated FeNO levels are one predictor of a positive response.

FeNO Levels

FeNO levels are currently measured in physician offices. Patients exhale completely and then take a normal breath in through a disposable filter. FeNO levels are then available in a few minutes for interpretation by your physician.

Normal, intermediate, and elevated levels are presented in the table below.

FeNO Levels In Adult and Children

 12 and older4–12
Low< 25< 20
Intermediate≥ 25- ≤ 50≥ 20- ≤ 35
High> 50>35
ppb= parts per billion

FeNO Use In Asthma Care

The Optimum Patient Care Research Data is the United Kingdom respiratory database that contains anonymous, long-term outcomes data for asthma patients in the UK. The database was examined to identify patients not receiving inhaled steroid therapy but might benefit and patients who were being managed medically using FeNO as a marker.

Investigators found that small numbers of primary care physicians treating asthma using FeNO. Interestingly, use of FeNO assessment was associated with increased adherence to a treatment regimen in addition to its use to initiate inhaled steroid therapy or an indicator for a step-up therapy. Additionally, other research has demonstrated the rate of asthma exacerbations to be lower in patients using FeNO as part of their monitoring strategy. A U.S. Based study at 10 asthma centers found that FeNO use led to higher doses of inhaled steroids compared to standard management based on asthma guidelines, but did provide significant improvements in asthma symptoms that were clinically significant.


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  7. Dwelk RA, Boggs PB, Erzurum SC et al. An Official ATS Clinical Practice Guideline: Interpretation of Exhaled Nitric Oxide Levels (FENO)for Clinical ApplicatiAccessed November 20, 2014.

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