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Why is FeNO higher in asthma patients?

Several major guidelines recommend testing FeNO to accurately assess Type 2 airway inflammation, a key characteristic of asthma.1-7 In fact, a FeNO level greater than 40 parts per billion (ppb) means asthma is seven times more likely,8 making FeNO a useful biomarker in suspected cases.9 But why is FeNO higher in asthma patients? This article looks at the science.

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Type 2 inflammation is mainly driven by interleukins 4, 5 and 13, also known as IL-4, IL-5 and IL-13. These cytokines are involved in the recruitment and production of immunoglobulin E (IgE), eosinophils and nitric oxide (NO), which is then released in exhaled breath as FeNO (fractional exhaled nitric oxide).10

The body naturally produces an amount of NO and low levels of exhaled NO are normal.1 When Type 2 airway inflammation is present (the type of inflammation responsible for up to 93% of asthma cases11), IL-4 and IL-13 upregulate the activity of the iNOS enzyme, which produces NO in the airway.10 Levels of exhaled NO increase, giving healthcare professionals an objective measure of airway inflammation.1

FeNO has now become a common biomarker of Type 2 airway inflammation, along with sputum and blood eosinophils.12 Studies have shown that both FeNO and blood eosinophils have a significant positive relationship (p<0.0001), with both accurately predicting eosinophilic airway inflammation.13 Furthermore, Dr Reynaud Panettieri, Vice Chancellor for Clinical and Translational Science and Director of the Rutgers Institute for Translational and Medical Sciences in New Jersey, US, recently told the crowd at a NIOX® webinar that knowing a patient has either high FeNO or high eosinophils predicts a 25% increase in exacerbations. With both high FeNO and high eosinophils, the increase was almost four times greater.14

However, unlike sputum and blood eosinophil procedures, testing for FeNO is simple and non-invasive, making it a convenient starting point for assessing airway inflammation right there at the patient’s appointment.15

What is “high FeNO”?

The American Thoracic Society (ATS) defines high FeNO as a test result over 50 ppb in adults and 35 ppb in children. This kind of level indicates that airway inflammation is present, supports an asthma diagnosis in patients with variable cough, wheeze and shortness of breath, and suggests that the patient is likely to respond to treatment with corticosteroids.1

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FeNO is not only useful in the diagnosis of asthma but also in its ongoing management, offering a potentially precision-based approach to patient care.16 One of the additional benefits of FeNO testing is the fact that the test is reproducible: being able to track patients’ FeNO levels over time means physicians can check how the treatment plan is panning out.15 When medication is working, FeNO levels usually fall.17

Performing a FeNO test is straightforward17 and can be completed within a couple of minutes at the patient’s appointment. There is no need for onward referral before FeNO and physicians can take immediate action should they find a patient’s treatment is not working as expected.

To find out more about FeNO testing in asthma and how NIOX VERO® can help you assess airway inflammation at the point-of-care, book a demo or check out our videos to see NIOX® technology in action.

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14. Dr Reynold Panettieri. Utility of FeNO and Biomarkers in Asthma. Available at:
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