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What is a normal FeNO value?

Measuring FeNO is straightforward and gives results at the point-of-care. The American Thoracic Society sets out clear cut-off points defining low, normal and high FeNO. But where do you begin? Should every patient start from the same baseline? Actually, FeNO is best applied within the context of the individual. After all, personalised treatment is important for better outcomes so being able to use FeNO to tailor asthma treatment is a distinct advantage. This article looks at how.

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“Is there a normal FeNO value?” the ATS asks. The guidelines divide the clinical cut-off points that it recommends into low, medium and high FeNO but it’s not a one-size-fits-all approach.1 Instead, the ATS also gives context for use when interpreting FeNO results and advice on how to distil the differences between patients into tailored treatment plans. First, let’s look at the variations between the recommended ranges and how the ATS views the patients in each.

Low FeNO (<25 ppb in adults/<20 ppb in children)

Low FeNO indicates that airway inflammation and responsiveness to corticosteroids are less likely. The ATS points out that, in patients presenting with non-specific respiratory symptoms, low FeNO suggests alternative diagnoses which will not be affected by an increase in steroid therapy. Such alternatives include rhinosinusitis, chronic obstructive pulmonary disease (COPD), bronchiectasis and primary ciliary dyskinesia (PCD). In asymptomatic asthma patients, low FeNO usually indicates that the disease is well controlled with a low risk of exacerbations.1

Intermediate FeNO (25-50 ppb in adults/20-35 ppb in children)

FeNO in the middle range should be interpreted cautiously and with reference to the clinical context. The ATS stresses that the weight placed on an intermediate FeNO result depends on whether the test is being used for diagnosis in a symptomatic, steroid-naïve patient or whether the patient’s FeNO has increased or decreased from a previous value by a clinically significant amount during monitoring over the longer term. You can find out more about interpreting variations in FeNO over time here.

High FeNO (>50 ppb in adults/>35 ppb in children)

High FeNO indicates airway inflammation is present and that a response to corticosteroids is likely. Again, this cut-off point was based on the results of pragmatic studies, the ATS says, highlighting that symptomatic, steroid-naïve patients with high FeNO are more likely to show responsiveness to inhaled corticosteroid (ICS) therapy irrespective of the diagnostic label (asthma or non-asthma).

Context is key

One of the most important considerations when interpreting patients’ FeNO levels is context. The ATS used specific rationale when selecting the cut-off points and says it’s vital to consider results in relation to the clinical setting.

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FeNO values can be affected by several factors, including gender, age, height, weight, genetics, smoking status, diet, atopy and anti-inflammatory medicines. For example, atopic individuals have higher FeNO levels while smokers tend to have lower.

Given the confounding factors, the ATS committee felt it would be more relevant to identify clinically meaningful FeNO cut-off points instead of reference values. The most important aspect is whether the patient has current respiratory symptoms or a prior diagnosis of airways disease: this helps to define the individual context.

Avoiding trial and error

The predictive values for FeNO (usually at cut-off points >25 ppb) were shown to be sufficiently robust to be used in the context of diagnosis – more than for conventional measurements such as peak flows and spirometry, and similar to those associated with bronchial challenge tests. In patients presenting with variable cough, wheeze, and shortness of breath, the ATS states that increased FeNO provides supportive evidence for an asthma diagnosis. Importantly, the potential of FeNO lies in its ability to identify steroid responsiveness. This is particularly helpful because it enables physicians to bypass an experimental “trial of steroids” or unnecessary long-term corticosteroid treatment.

Every patient is different so what’s “normal” for one won’t be the same for another. It’s crucial to take into account all the variables in each case and interpret FeNO results accordingly. Knowing a FeNO device is precise and dependable is a good starting point so physicians can trust that the results truly reflect the individual.

Famous for its accuracy and reliability, NIOX® technology was one of the lynchpins in the development of the ATS cut-off points for FeNO interpretation, meeting all the success criteria. Indeed, the cut-offs were derived from a large number of studies where 83% of participants used NIOX® devices. As such, NIOX® is most likely to reflect the ATS guidelines. NIOX VERO® is also the only device in the UK that is fully compliant with the ATS and European Respiratory Society recommendations for the standardisation of FeNO measurement.

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1. Dweik RA et al. An official ATS clinical practice guideline: Interpretation of exhaled nitric oxide levels (FeNO) for clinical applications. Am J Respir Crit Care Med. 2011;184(5):602-15.