Fractional exhaled nitric oxide FeNO testing is one of the most effective ways to measure airway inflammation in asthma. When FeNO levels are raised, it provides clinicians with valuable insight into what is happening in the airways at the time of testing. But what exactly does a high FeNO reading mean, and how should clinicians interpret it alongside symptoms, lung function and clinical history?
A high FeNO result does not diagnose asthma alone. Instead, it indicates that eosinophilic or type 2 inflammation is present, and that symptoms are likely to respond well to inhaled corticosteroid therapy.
Understanding FeNO readings
FeNO values are expressed in parts per billion ppb. While exact interpretation varies by age, guidance generally categorises results into low, intermediate and high ranges.
Typical FeNO guidance levels
Adults:
- Low: under 25 ppb
- Intermediate: 25 to 50 ppb
- High: above 50 ppb
Children (5–16):
- Low: under 20 ppb
- Intermediate: 20 to 35 ppb
- High: above 35 ppb
What a high FeNO level suggests
A raised FeNO level indicates that eosinophilic inflammation is active in the airways. This finding is particularly important in patients with symptoms suggestive of asthma.
High FeNO suggests:
- Airway inflammation consistent with asthma
- Strong likelihood of steroid responsiveness
- Increased risk of exacerbation if untreated
- Need for closer monitoring or earlier intervention
For many clinicians, high FeNO gives confidence to start or escalate inhaled corticosteroid therapy if clinically appropriate.
High FeNO and treatment response
Patients with raised FeNO levels often benefit from inhaled corticosteroids (ICS) because this medication targets type 2 inflammation directly.
A high FeNO reading can support:
- Initiating ICS when diagnosis remains uncertain
- Increasing dose or adding preventer therapy
- Confirming that previous ICS withdrawal was premature
- Guiding shared decision making about medication options
Conversely, if FeNO remains high despite treatment, clinicians may need to explore adherence, inhaler technique or co existing triggers such as allergens.
Why high FeNO is not enough on its own
A high FeNO reading is significant, but it should never be interpreted in isolation. Symptoms, spirometry, history and patient context remain important.
FeNO does not diagnose:
- COPD
- Vocal cord dysfunction
- Hyperventilation syndromes
- Non eosinophilic asthma
This is why combining FeNO with spirometry and clinical assessment provides a clearer diagnostic picture than symptoms alone.
Non-asthma causes of high FeNO
Raised FeNO can occasionally occur in the absence of asthma. Contributing factors include:
- Seasonal allergen exposure (grass, pollen, dust mites)
- Viral respiratory infections
- Poor inhaler adherence
- Dietary nitrate intake (less common but recognised)
- Atopy or allergic rhinitis
Understanding these drivers helps clinicians avoid unnecessary escalation and instead address root causes.
Using high FeNO to prevent deterioration
Patients with elevated FeNO are at higher risk of future loss of control, particularly during viral seasons or allergen peaks. Measuring FeNO regularly enables clinicians to spot a rising trend before symptoms worsen.
This supports:
- Early intervention before urgent care attendance
- Better control during hay fever season
- Tailored asthma action planning
- Reduced emergency steroid bursts
High FeNO can therefore shape anticipatory care rather than reactive response.
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