Interpreting FeNO test results

Asthma diagnosis: interpreting FeNO using cut-off points

The ATS uses the cut-off points above to indicate the likelihood of responsiveness to anti-inflammatory treatment such as corticosteroids.⁷ Low FeNO is associated with a low likelihood of airway inflammation and therefore a reduced expectation of responsiveness.⁷ High FeNO, on the other hand, means airway inflammation is present and responsiveness to corticosteroids would be likely.⁷ Where FeNO values fall in between, the ATS recommends interpreting the patient's results within the clinical context for a fuller picture.⁷

Some countries have produced guidelines with a single cut-off point instead of a range.⁸ The UK's BTS/SIGN/NICE joint guidelines released in November 2024, for example, suggests a FeNO score of 35 ppb in children and 50 pbb in adults as the unique cut-off to support an asthma diagnosis.⁸ FeNO testing is the first front-line diagnostic tool recommended for asthma in both adults and children in these guidelines.⁸ It is noted to be particularly valuable as an effective and patient-friendly method for confirming an asthma diagnosis.⁸ Through this ‘rule-in’ approach the committee agreed upon the importance of personalised action plans.⁸ With FeNO testing being recommended as part of regular reviews, as well as before and after any change in asthma therapy.⁸

Airway inflammation is a key characteristic of asthma and assessing its severity with FeNO testing can help to achieve more accurate diagnosis.^1,2 When FeNO is 40 ppb or more, a patient could be seven times more likely to have asthma.⁹

Asthma management: interpreting FeNO using variation from baseline It is important to note that the baseline can vary according to whether the patient is currently receiving inhaled corticosteroids (ICS) as ICS reduce FeNO levels.^1,7 At follow-up appointments, the ATS guidelines recommend that an increase in FeNO greater than 20% for baseline values over 50 ppb or more than 10 ppb for values lower than 50 ppb should be considered a significant increase.⁷ A reduction of at least 20% in FeNO for values over 50 ppb or more than 10 ppb for values lower than 50 ppb indicate a significant response to anti-inflammatory therapy.⁷

Asthma affects a large number of people, many of whom may not be aware they have it, especially if their symptoms aren’t severe.¹⁰ High FeNO is associated with over three times greater risk of potentially severe asthma exacerbations.¹ An elevated FeNO result can open up conversation with patients about inhaler technique and adherence or guide physicians in titrating corticosteroid dosage.^4,11 Optimising treatment using FeNO has been shown to help reduce exacerbations by up to 50%.^4,12,13

FeNO-guided asthma management not only helps to improve outcomes for patients but is also associated with reduced costs, with several studies citing that integrating FeNO testing into patient care can ensure that medication is optimised, and can reduce or avoid unscheduled visits to the doctor or admissions to hospital.^4,14-16 The BTS/SIGN/NICE guidelines also agree that FeNO tests present an opportunity within primary care as a routine measurement.⁸ Further highlighting their usefulness within clinical practice in increasing the accuracy of asthma diagnosis and cost-effectiveness over time.⁸

Factors influencing FeNO

Some factors can influence FeNO levels. Particularly, FeNO testing should always be performed prior to spirometry because of a potential and temporary reduction of FeNO.⁶ In addition, there are other influencing factors. Although they are not usually clinically significant, it is recommended that patients avoid food, drink, exercise and smoking before testing FeNO.^6,7

If you would like to know more about what your patients FeNO scores mean, head to the the FeNO learn app where you will find all our FeNO educational resources, Including our FeNO Interpretation calculator which gives you recommendations based off the latest guidelines.