Key learning points

  • Fraction of exhaled nitric oxide (FeNO) is a gas that is produced by the respiratory mucous membrane and is raised in the presence of eosinophilic inflammation in the airways
  • Measuring FeNO in patients presenting with a history suggestive of asthma is a useful objective test to perform as a part of the diagnostic jigsaw
  • As with any investigation, the result needs clinical interpretation considering the individual situation


The GINA definition of asthma highlights the inflammatory nature of the disease as well as some of the characteristic respiratory symptoms such as wheeze or shortness of breath.1 When making a diagnosis of asthma, a clear history of symptoms associated with the disease leading to a measurement of lung function using spirometry or serial peak flow has sometimes led to a formal diagnosis being made.

However, the underlying pathology of most asthma is of eosinophilic airway inflammation yet there has been little focus on measuring this inflammation to add to the clinical decision making resulting in a diagnosis.

Performing FeNO testing

FeNO testing is a quick simple test that can be performed by most patients following clear instructions. There are several devices available for FeNO testing, which involve the patient blowing into the monitor for 6–10 seconds at a steady flow rate of 50ml/second. At the end of the test the results are displayed on the device in parts per billion (ppb).

What do the results mean?

Different guidelines suggest different reference ranges for interpretation. The Irish College of General Practitioners (ICGP) suggest FeNO levels above 40ppb (in adults) are high and increases the likelihood of asthma being the correct diagnosis.2 GINA describes a high FeNO level of >50ppb in non-smokers may distinguish an asthma diagnosis from a COPD diagnosis, as FeNO levels are usually normal (or low in current smokers) for those with COPD.1 Bjermer et al suggests three FeNO reference ranges: Below 25ppb (below 20ppb in children) the low range, 25–50ppb (20–35ppb in children) the intermediate or elevated range, and >50ppb (>35ppb in children) the high range.3 This gives more scope for clinical judgement and interpretation.

A raised FeNO level, in itself, is not sufficient to diagnose asthma. It is a marker of eosinophilic airway inflammation. For an accurate diagnosis of asthma to be reached, the clinician needs to take a detailed clinical history looking for pointers that would raise a suspicion of asthma and to review the medical records. FeNO testing does not replace other tests performed to investigate a potential diagnosis of asthma – it is an additional part of the diagnostic jigsaw puzzle.

Guidelines for the management of diagnosed asthma 

GINA guidelines on the management of asthma include FeNO testing as one of the possible additional tests in cases where patients have additional risk factors for exacerbations even if they have only a few asthma symptoms.1 There is some evidence showing FeNO>50 parts per billion (ppb) was associated with a good short-term response to inhaled corticosteroids (ICS) in non-smoking patients, but the long-term risk of exacerbations were not evaluated.1 The ICGP agree that FeNO measurement may help to guide ICS dose but that the role of FeNO testing within primary care is yet to be established.2


Asthma is a condition that frequently causes chronic eosinophilic inflammation of the airways. This results in an increase in the production of exhaled nitric oxide gas, which can be measured quickly and easily to assess airway inflammation and to provide objective evidence to assist clinical decision making when diagnosing asthma. Cost may be perceived as a barrier but improved diagnosis and personalised tailored care can result in cost savings.

Carol Stonham is a senior nurse practitioner (respiratory) at Gloucestershire CCG.

This project was initiated and funded by Teva Respiratory. Teva have had no influence over content. Topics and content have been selected and written by independent experts.