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
  • FeNO testing can be easily performed in primary care settings in patients from the age of five years


Key definitions of asthma highlight the inflammatory nature of the disease. The British Thoracic Society (BTS) and the Scottish Intercollegiate Guidelines Network (SIGN) describes airway hyper-responsiveness and inflammation as components of the disease,1 while the Global Initiative for Asthma (GINA) focuses on the chronic inflammatory disorder.2 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.

Guidelines for diagnosis

BTS/SIGN guidelines on the management of asthma include FeNO testing as one of the possible additional tests in cases where there is an intermediate probability of a person having asthma.1 NICE released guidelines in 2017 (updated in 2021) on the diagnosis and management of chronic asthma.3 NICE suggests that all adults aged 17 and over should be offered a FeNO test as well as spirometry (with reversibility if the test shows obstruction) if a diagnosis of asthma is being considered.3 The recommendation for children (aged 5–16) is that they should all perform spirometry with reversibility for obstructive results, and that if this result is inconclusive FeNO should also be performed.3 This guideline addresses the importance of measuring both airway obstruction and inflammation when making a diagnosis of asthma.

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 in the UK, they 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. NICE guidelines suggest a simplified cut off of 0–40ppb (0–35ppb aged 5–17 years) as normal and FeNO levels above 40ppb (or 35ppb in children) increasing the likelihood of asthma being the correct diagnosis.3 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.4 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.


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.

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