How Is Asthma Diagnosed in the UK? Spirometry, Peak Flow and FeNO Explained
▶ Asthma diagnosis UK
Asthma diagnosis in the UK follows NICE NG245 and requires objective evidence of variable airflow obstruction. Key tests are: spirometry with bronchodilator reversibility (FEV1 improvement ≥12% and ≥200ml after salbutamol); FeNO testing (≥40 ppb supports allergic asthma); and peak flow variability monitoring (≥20% over 2–4 weeks). Clinical symptoms alone are not sufficient for diagnosis.
Asthma diagnosis requires more than a symptom description — it requires objective evidence of variable airflow obstruction. This is because asthma symptoms (wheeze, breathlessness, chest tightness, cough) are non-specific and shared by several other conditions. NICE NG245 (2024) establishes a clear diagnostic pathway for adults and young people that combines clinical history with objective tests. This guide explains each test, what the results mean, and what happens after diagnosis.
For an overview of asthma symptoms and types, see: Asthma Explained: Symptoms, Triggers, Causes & Treatment.
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Start Consultation →Why Asthma Diagnosis Requires Objective Testing
Many conditions cause wheezing and breathlessness. Without objective testing, over-diagnosis is common — leading to unnecessary inhaler prescriptions and missed alternative diagnoses. Under-diagnosis is equally common, particularly in adults and older people, where asthma is sometimes attributed to ageing or deconditioning.
NICE NG245 requires that, wherever possible, at least one objective test is performed before an asthma diagnosis is confirmed. The test types are chosen because they measure the key physiological feature of asthma: variable, reversible airflow obstruction.
NICE NG245 Diagnostic Pathway
The NICE NG245 pathway for adults and young people (≥5 years) proceeds through structured stages:
- Symptom assessment — characteristic pattern of wheeze, cough, breathlessness and chest tightness; variability over time; response to previous treatment; triggers identified
- Risk stratification — assess probability of asthma as high, intermediate, or low based on symptoms, atopic history, and examination findings
- Objective testing — FeNO first in adults; spirometry with bronchodilator reversibility; PEF monitoring if spirometry normal at time of testing
- Diagnosis and initial management — diagnosis confirmed, treatment initiated per NICE stepwise approach, and monitoring plan established
For treatment steps following diagnosis, see: Types of Asthma Treatment Explained: From Blue Inhalers to Long-Term Control.
Spirometry: What It Is and What the Results Mean
Spirometry measures the volume and speed of air you can exhale. Two key measurements are taken:
- FEV1 (Forced Expiratory Volume in 1 second) — how much air you can exhale forcefully in the first second
- FVC (Forced Vital Capacity) — the total volume exhaled during a maximum effort breath
An FEV1/FVC ratio of less than 0.7 (70%) indicates obstructive airflow limitation — consistent with asthma. To test reversibility, the measurement is repeated 15–20 minutes after inhaling salbutamol (reliever inhaler). A positive bronchodilator reversibility result is defined as:
- FEV1 improvement of ≥12% AND ≥200ml compared to the pre-bronchodilator reading
A positive reversibility test strongly supports an asthma diagnosis. Normal spirometry at the time of testing does not exclude asthma — the condition is variable and the airways may be normal between episodes.
FeNO (Fractional Exhaled Nitric Oxide) Test
FeNO is a simple, non-invasive test that measures the level of nitric oxide in exhaled air. Nitric oxide is produced by eosinophils — the inflammatory cells involved in allergic asthma. Elevated FeNO indicates active eosinophilic airway inflammation and predicts a good response to inhaled corticosteroid (ICS) preventer therapy.
| FeNO Result | Interpretation (Adults) |
|---|---|
| <25 ppb | Low — eosinophilic inflammation unlikely; ICS less likely to be beneficial |
| 25–39 ppb | Intermediate — consider in context of other tests |
| ≥40 ppb | High — supports diagnosis of allergic asthma; ICS treatment likely to be beneficial |
NICE NG245 recommends FeNO testing as part of the adult asthma diagnostic assessment. The test is quick (exhale into a device over 10 seconds), non-invasive, and widely available in GP surgeries and respiratory clinics. Smoking, food intake, and recent respiratory infection can affect results.
Peak Expiratory Flow (PEF): Home Monitoring
Peak flow measures the fastest speed at which you can exhale. A peak flow meter is a simple handheld device. NICE recommends using peak flow monitoring over 2–4 weeks to assess variability:
- Record PEF twice daily — morning on waking and evening before bed
- Variability of ≥20% across readings supports a diagnosis of asthma
- Peak flow is typically lowest in the early morning — a pattern characteristic of asthma
Peak flow monitoring is particularly useful when spirometry is normal at the time of testing, because the variability of asthma means airflow may be normal between attacks. It is also used in ongoing monitoring and as part of a written asthma action plan.
Bronchial Challenge Testing
If spirometry and FeNO are inconclusive but asthma is still suspected, bronchial challenge testing (also called bronchial provocation testing) may be performed in a respiratory clinic. Common methods include:
- Methacholine challenge — inhaling increasing doses of methacholine to provoke airway narrowing in susceptible airways. A positive result (PC20 ≤8 mg/ml) indicates airway hyperresponsiveness consistent with asthma.
- Exercise challenge — spirometry performed before and after a standardised exercise protocol. Useful for diagnosing exercise-induced bronchoconstriction.
- Mannitol challenge — alternative to methacholine; similar interpretation.
Diagnosing Asthma in Children
Asthma diagnosis in young children (under 5) is particularly challenging because spirometry requires sustained effort. Diagnosis in this age group relies more heavily on symptom pattern, clinical assessment, and response to treatment. NICE NG245 provides separate guidance for children under 5, emphasising a trial of treatment and careful reassessment. Children aged 5 and over can generally complete spirometry and FeNO testing.
Access Doctor’s online asthma service is for adults (18+) only. Children with suspected or confirmed asthma should be managed by their GP or paediatric respiratory team.
Asthma vs Other Conditions That Cause Wheeze
| Condition | Key Distinguishing Features |
|---|---|
| COPD | Fixed (irreversible) airflow obstruction; strong smoking history; FEV1/FVC <0.7 that does not reverse with bronchodilator; FeNO usually normal |
| Cardiac failure | Breathlessness worse lying flat (orthopnoea); raised JVP; ankle oedema; basal crackles on chest examination; ECG and echo abnormalities |
| Vocal cord dysfunction | Stridor (inspiratory wheeze); normal spirometry between episodes; often triggered by stress; laryngoscopy confirms |
| Hyperventilation syndrome | Tingling in fingers and face; dizziness; normal spirometry and FeNO; responds to breathing retraining |
| Bronchiectasis | Chronic productive cough; CT chest shows dilated bronchi; usually associated with recurrent infections |
What Happens After Diagnosis?
Following confirmed asthma diagnosis, NICE recommends initiating NICE-stepwise treatment. Most newly diagnosed adults start at Step 1 (reliever inhaler only) or Step 2 (add preventer inhaler), depending on symptom frequency and severity. You will also be given:
- A written asthma action plan personalised to your peak flow and symptoms
- Inhaler technique training — see: Asthma Inhalers: Your Complete Guide
- An invitation for an annual asthma review
- Advice on trigger identification and avoidance
Access Doctor can support ongoing management of confirmed, stable asthma including repeat reliever and preventer inhaler prescriptions via a GPhC-regulated online consultation.
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Start Consultation →Frequently Asked Questions
How is asthma diagnosed in the UK?
Asthma is diagnosed in the UK following NICE NG245 guidance. Diagnosis requires both a clinical assessment (symptom history, variability, triggers) and objective testing. Key tests are: spirometry with bronchodilator reversibility (FEV1 improvement ≥12% and ≥200ml after salbutamol), FeNO testing (≥40ppb supports allergic asthma in adults), and peak expiratory flow (PEF) variability monitoring (≥20% variability over 2–4 weeks). No single test is diagnostic alone — the clinical picture is combined with test results.
Can asthma be diagnosed without spirometry?
NICE NG245 requires objective testing as part of the diagnostic pathway. While peak flow monitoring or FeNO can provide supporting evidence, spirometry with bronchodilator reversibility is the recommended primary objective test for adults. A clinical diagnosis without any objective testing is not recommended, as it risks both over-diagnosis (treating people who do not have asthma) and under-diagnosis (missing the condition).
What is a FeNO test for asthma?
FeNO stands for fractional exhaled nitric oxide. It measures the level of nitric oxide in exhaled air, which is elevated when there is eosinophilic (allergic) airway inflammation. Adults with FeNO ≥40 ppb are more likely to have allergic asthma and to respond well to inhaled corticosteroid (ICS) preventer therapy. FeNO is now recommended by NICE as part of the adult asthma diagnostic pathway. The test is quick, non-invasive, and performed in GP surgeries and respiratory clinics.
What peak flow variability indicates asthma?
A peak expiratory flow (PEF) variability of 20% or more across morning and evening readings over 2–4 weeks supports a diagnosis of asthma. The calculation is: ((highest reading – lowest reading) / highest reading) × 100. Significant variability — especially if readings are lowest in the morning — is consistent with the variable airflow obstruction that characterises asthma.
Can adults develop asthma and be diagnosed for the first time?
Yes. Adult-onset asthma is common, particularly in people aged 20–40. It may be triggered by occupational exposures (occupational asthma — important in bakers, hairdressers, painters), hormonal changes, respiratory infections, or exposure to new allergens. If you develop unexplained recurrent breathlessness, wheeze, chest tightness, or persistent cough, see your GP for formal spirometry-based assessment.
What conditions can be confused with asthma?
Several conditions cause similar symptoms to asthma: COPD (chronic obstructive pulmonary disease — common in smokers; irreversible obstruction), cardiac failure (breathlessness worse on lying flat), vocal cord dysfunction (breathlessness and wheeze, particularly on inspiration), bronchiectasis, and hyperventilation syndrome. Spirometry and FeNO testing help distinguish asthma from these alternatives. This is why objective testing matters — symptoms alone cannot reliably differentiate asthma from other conditions.
References
- NICE. Asthma: diagnosis, monitoring and chronic asthma management (NG245). 2024. nice.org.uk/guidance/ng245
- NHS. Asthma. nhs.uk/conditions/asthma
- Asthma + Lung UK. Asthma facts and statistics. asthma.org.uk
- BTS/SIGN. British Guideline on the Management of Asthma (SIGN 158). 2023. sign.ac.uk
- MHRA. Salbutamol (Ventolin Evohaler) summary of product characteristics. medicines.org.uk/emc
- GPhC. Standards for registered pharmacies. pharmacyregulation.org
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Asthma inhalers are prescription-only medicines — a clinical consultation is required before they can be dispensed. If you are experiencing a severe asthma attack, call 999 immediately. Always follow the guidance of your prescriber or asthma nurse. In a medical emergency, call 999.