FeNO Cut-off Values for Asthma Diagnosis
For adults, a FeNO value >50 ppb indicates eosinophilic airway inflammation and likely asthma, while <25 ppb makes eosinophilic inflammation and asthma unlikely. For children, the cut-off values are >35 ppb and <20 ppb, respectively. 1
FeNO Cut-off Values by Age Group
Adults:
- Low FeNO: <25 ppb - Eosinophilic airway inflammation unlikely
- Intermediate FeNO: 25-50 ppb - Interpret with caution
- High FeNO: >50 ppb - Eosinophilic airway inflammation present
Children:
- Low FeNO: <20 ppb - Eosinophilic airway inflammation unlikely
- Intermediate FeNO: 20-35 ppb - Interpret with caution
- High FeNO: >35 ppb - Eosinophilic airway inflammation present
Clinical Significance of FeNO Values
High FeNO (>50 ppb in adults, >35 ppb in children):
- Strongly indicates presence of eosinophilic airway inflammation 1, 2
- Predicts likely responsiveness to inhaled corticosteroid (ICS) therapy 1
- Associated with increased risk of asthma exacerbations 2, 3
- Has a positive predictive value of 77% for sputum eosinophilia ≥3% 4
Low FeNO (<25 ppb in adults, <20 ppb in children):
- Eosinophilic airway inflammation unlikely 1
- Alternative diagnoses should be considered 1
- Negative predictive value of 88% for ruling out sputum eosinophilia 4
- Patient unlikely to benefit from ICS therapy 1
Intermediate FeNO (25-50 ppb in adults, 20-35 ppb in children):
- Interpret with caution and in clinical context 1
- Monitor changes in FeNO over time 1
- May indicate atopic asthma, eosinophilic bronchitis, or COPD with mixed inflammatory phenotype 1
Interpreting FeNO in Clinical Practice
Diagnostic Value:
- FeNO >50 ppb has high specificity but moderate sensitivity (37%) for detecting eosinophilic inflammation 4
- FeNO should not be used as the sole diagnostic criterion for asthma 2
- Combining FeNO with other clinical assessments (like Asthma Control Test) significantly improves diagnostic accuracy 5
Monitoring Value:
- Persistent elevation of FeNO ≥50 ppb over time is associated with higher exacerbation risk 3
- High variability in FeNO measurements over time also predicts exacerbations 3
- A significant response to anti-inflammatory therapy is indicated by:
- A reduction of at least 20% in FeNO for values over 50 ppb
- A reduction of more than 10 ppb for values lower than 50 ppb 1
Important Considerations and Pitfalls
Confounding Factors:
- Age, height, sex, and atopy status can affect FeNO levels 1
- Smoking reduces FeNO levels, potentially leading to false-negative results 1
- Recent respiratory infections can temporarily increase FeNO 2
Clinical Pitfalls:
- Not all asthma is associated with high FeNO (e.g., non-eosinophilic/neutrophilic asthma) 2
- Overreliance on absolute FeNO values without considering clinical context 2
- Intermediate FeNO values (25-50 ppb) are common in real-life populations, which may limit clinical usefulness 4
- FeNO should be interpreted alongside clinical symptoms and lung function tests, not in isolation 1, 2
By understanding these cut-off values and their clinical significance, clinicians can better utilize FeNO measurements to diagnose asthma, predict steroid responsiveness, and monitor treatment efficacy.