Asthma diagnosis in primary care is rarely straightforward. Symptoms can change over time, patients may have variable lung function and clinical presentations overlap with other respiratory conditions. To support confident decisions, many clinicians now use a combination of FeNO testing and spirometry to build a more accurate picture of airway health.
FeNO and spirometry each provide valuable but different information. When used together, they improve diagnostic certainty, guide treatment and ensure patients receive the right support as early as possible.
What is FeNO and what does it measure
FeNO stands for fractional exhaled nitric oxide. It measures the concentration of nitric oxide in the breath, which rises when inflammation is present in the airways. A simple, slow exhalation into a handheld analyser provides a FeNO reading in parts per billion.
FeNO is one of the only rapid tests capable of showing whether type 2 airway inflammation is active at the time of assessment.
What spirometry measures
Spirometry assesses lung function by measuring how much air a patient can exhale and how quickly they can blow it out. Readings such as FEV1, FVC and the FEV1/FVC ratio help clinicians understand airflow limitation and obstruction.
Unlike FeNO, spirometry does not provide information on airway inflammation. It measures mechanical function rather than inflammatory activity.
Spirometry is:
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Effort dependent
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Symptom influenced
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Sometimes normal between episodes for asthma patients
This can make interpretation challenging in early or variable disease.
Why these tests are complementary
FeNO and spirometry work best when used together, because they provide insight into two different aspects of respiratory health.
FeNO shows:
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Likelihood of steroid responsiveness
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Whether symptoms are inflammation driven
Spirometry shows:
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Airflow obstruction
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Bronchial narrowing
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Reversibility where bronchodilators are used
Combining both gives clinicians greater clarity and helps avoid misdiagnosis, particularly when symptoms are variable or non-specific.
Supporting diagnostic pathways
Current respiratory guidelines recognise both FeNO and spirometry as important components of asthma assessment. When spirometry results are normal or borderline, FeNO can uncover inflammation that may otherwise be overlooked.
Similarly, a raised FeNO result alongside reduced lung function strengthens the case for asthma and helps guide timely treatment.
Using both tests supports clinical confidence when:
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Symptoms are mild or intermittent
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Asthma is suspected but unconfirmed
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Alternative diagnoses need ruling out
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Paediatric patients struggle with spirometry
Using FeNO and spirometry in primary care
FeNO is quick, non invasive and suitable for most patients aged five and above. Spirometry requires trained staff and patient effort, but remains an essential measure of respiratory status.
Together, they enable:
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Faster diagnosis
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Clearer treatment decisions
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Fewer unnecessary steroid prescriptions
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More appropriate referrals to secondary care
Many NHS GP practices and community clinics are already embedding FeNO alongside spirometry to standardise care and reduce variation between services.
FeNO for monitoring beyond diagnosis
Spirometry remains useful for monitoring disease progression and confirming airflow limitation, while FeNO provides valuable insight into ongoing inflammatory activity.
FeNO can:
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Detect non-adherence to inhaled corticosteroids
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Identify flare ups early
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Prevent unnecessary escalation of therapy
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Support shared decision making
This makes the combination of both tools particularly beneficial for long term management and annual asthma reviews.
Better outcomes for patients and services
By providing rapid access to both inflammatory and functional data, clinical teams can make more confident decisions and reduce diagnostic uncertainty.
For patients, this means:
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Faster access to appropriate treatment
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Fewer avoidable asthma attacks
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Reduced hospital admissions
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Improved engagement and confidence in their care
For services, it supports:
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More efficient appointment use
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Reduced repeat consultations
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Better medication stewardship
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Greater alignment with national standards
In summary
FeNO and spirometry are not competing tests. They are vital, complementary tools that together provide a much clearer insight into asthma and airway health. Primary care teams who use both are better equipped to diagnose accurately, tailor treatment and improve outcomes for the patients they support.
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