Originally published: September 2018

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

Introduction

Key definitions of asthma highlight the inflammatory nature of the disease. BTS/SIGN describes airway hyper-responsiveness and inflammation as components of the disease,1 while 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 fractional exhaled nitric oxide (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 on the diagnosis and management of chronic asthma. 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 the history and examination is suggestive of asthma.3 The recommendation for children (aged 5–17) 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 instruction. There are currently three manufacturers with devices available in the UK. All involve the patient blowing into the monitor for 6–10 seconds at a steady flow rate of 50ml/second, which on all devices has a visual to guide the flow rate. 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.

Summary

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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