How Is Asthma Diagnosed in the UK? Spirometry, Peak Flow and FeNO Explained
The tests used to diagnose asthma under the 2024 NICE, BTS and SIGN guideline (blood eosinophils, FeNO, spirometry, peak flow and challenge tests), what the results mean, and what happens next.
Part of our asthma guide: symptoms, triggers and treatment.
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The short version: asthma is diagnosed by your GP or asthma nurse from your symptoms plus objective tests. Under the 2024 NICE, BTS and SIGN guideline (NG245), adults usually start with a blood eosinophil count or FeNO breath test, then spirometry before and after a reliever, then 2 weeks of peak flow readings, and finally a bronchial challenge test if it’s still unclear. Asthma can’t be diagnosed online.
Already diagnosed? See our asthma inhalers online.
Who our online service is for. Access Doctor supplies asthma and COPD inhalers to adults aged 18 and over who already have a diagnosis, for the same inhaler, strength and dose they’re prescribed now, and we share the consultation with your GP. We can’t diagnose asthma or start or change treatment online. Children’s inhalers are managed by their GP or asthma nurse.
Diagnosing asthma takes more than a description of symptoms. Wheeze, breathlessness, chest tightness and cough are shared by several other conditions, so UK guidance asks for objective tests that show the variable, reversible airway inflammation and narrowing typical of asthma. This guide walks you through each test, what the results mean and what happens after diagnosis. For symptoms and types of asthma, see our asthma guide.
Why Asthma Diagnosis Requires Objective Testing
Without tests, asthma gets both over-diagnosed — inhalers people don’t need, and other conditions missed — and under-diagnosed, especially in adults and older people whose symptoms are put down to age or being unfit.
The 2024 NICE, BTS and SIGN guideline (NG245) sets out which tests to use and in what order. A test result only confirms asthma when your history and examination also point to it, and many people need more than one test.
NICE NG245 Diagnostic Pathway
For adults (17 and over) whose symptoms suggest asthma, the tests are used in this order, moving on only if the previous one doesn’t confirm asthma:
1
Blood eosinophils or FeNO
Asthma is diagnosed if the eosinophil count is above the laboratory’s normal range, or FeNO is 50 ppb or more.
2
Spirometry with bronchodilator reversibility
Asthma is diagnosed if FEV1 rises by 12% or more and 200ml or more after a reliever, or by 10% or more of your predicted normal FEV1.
3
Peak flow variability
If spirometry isn’t available or is delayed: peak flow twice a day for 2 weeks. Variability of 20% or more supports asthma.
4
Bronchial challenge test
If asthma is still suspected, you’re referred for a test of how sensitive your airways are. Asthma is diagnosed if they’re over-responsive.
After diagnosis, treatment follows the NG245 pathway, usually starting with a combination inhaler used as needed (AIR). Our asthma inhalers guide explains it.
Blood Eosinophil Count
Eosinophils are white blood cells involved in a lot of asthma inflammation. A simple blood test from your GP practice measures them. A count above the laboratory’s normal range, in someone whose symptoms suggest asthma, supports the diagnosis. Steroid inhalers and tablets can lower the count — which is why these tests are best done before treatment starts.
FeNO (Fractional Exhaled Nitric Oxide) Test
FeNO measures nitric oxide in the air you breathe out. It’s raised when there’s eosinophilic (“type 2”) inflammation in the airways — the kind that responds well to inhaled steroids.
| Who | FeNO result that supports asthma (NG245) |
|---|---|
| Adults (17 and over) | 50 ppb or more |
| Children and young people aged 5 to 16 | 35 ppb or more |
You breathe out slowly and steadily into a handheld machine for about 10 seconds. It’s quick and painless, and available in many GP practices and community diagnostic centres. Recent smoking, some foods, a recent chest infection and inhaled steroids can all affect the result. A FeNO below the threshold doesn’t rule asthma out — the next test is used. FeNO also comes in handy later, to check whether asthma is controlled.
Spirometry: What It Is and What the Results Mean
Spirometry measures how much air you can breathe out and how fast. You take a deep breath in and blow out as hard and fast as you can into a mouthpiece. Two key measurements are:
- FEV1 (forced expiratory volume in 1 second): how much air you can blow out in the first second
- FVC (forced vital capacity): the total amount you can blow out in one breath
To test reversibility, the test is repeated about 15 minutes after a reliever (salbutamol). Under NG245, asthma is diagnosed in adults if FEV1 rises by 12% or more and 200ml or more, or by 10% or more of your predicted normal FEV1.
A low FEV1/FVC ratio shows airway narrowing, but on its own it can’t tell asthma apart from COPD — reversibility can. Normal spirometry doesn’t rule asthma out either, because asthma varies and your airways may simply be behaving on the day of the test.
Peak Expiratory Flow (PEF): Home Monitoring
Peak flow is the fastest speed you can blow air out. You measure it with a small handheld peak flow meter. Under NG245, peak flow is used when spirometry isn’t available or is delayed:
- measure it twice a day, morning and evening, for 2 weeks, taking the best of 3 blows each time
- variability of 20% or more supports asthma
- readings that are lowest in the early morning are typical of asthma
Your GP or asthma nurse works out the variability from your diary. Peak flow is also used to monitor asthma, and many action plans are based on your personal best reading.
Bronchial Challenge Testing
If asthma is still suspected after the other tests, you may be referred for a bronchial challenge (provocation) test, usually in a hospital or specialist service. You breathe in a substance — such as methacholine or mannitol — in increasing amounts while your breathing is measured. Airways that narrow easily (hyperresponsiveness) support asthma. For symptoms that only happen with exercise, an exercise challenge test may be used; see our exercise-induced asthma guide.
Could It Be Your Work?
Asthma that starts in adulthood, or gets worse, can be caused by something at work: flour, latex, isocyanates in spray paints, hairdressing chemicals or animal proteins, to name a few. A classic clue is symptoms that improve at weekends or on holiday. Tell your GP about your job — occupational asthma needs to be confirmed early, often with peak flow readings at and away from work, because continued exposure can cause lasting damage.
Diagnosing Asthma in Children
For children aged 5 to 16, NG245 starts with a FeNO test (35 ppb or more supports asthma), then spirometry with a reliever, then peak flow readings, and then allergy tests (a house dust mite skin prick test, or IgE and eosinophil blood tests), with a specialist referral if it’s still unclear. For children under 5, tests aren’t reliable, so diagnosis is based on symptoms and how they respond to a trial of treatment, with specialist referral for repeated hospital visits.
Access Doctor’s online asthma service is for adults (18 and over) only. Children with suspected or diagnosed asthma should be looked after by their GP, asthma nurse or children’s respiratory team.
Asthma vs Other Conditions That Cause Wheeze
| Condition | Key distinguishing features |
|---|---|
| COPD | Usually in current or ex-smokers over 35; narrowing that doesn’t fully reverse with a reliever; symptoms slowly worsen rather than coming and going |
| Heart failure | Breathlessness worse lying flat or at night; ankle swelling; tests such as a blood test (BNP) and heart scan |
| Inducible laryngeal obstruction (vocal cord dysfunction) | Noisy, tight breathing in (rather than out), often sudden; spirometry may be normal between episodes; diagnosed by a camera test of the voice box |
| Dysfunctional breathing or hyperventilation | Tingling in fingers or around the mouth, dizziness, sighing; normal tests; helped by breathing retraining |
| Bronchiectasis | Long-term cough with phlegm and repeated chest infections; diagnosed with a CT scan |
Some people have more than one condition, such as asthma and COPD, or asthma with dysfunctional breathing. Our guide to Ventolin for shortness of breath covers causes of breathlessness a reliever won’t help.
What Happens After Diagnosis?
Once asthma is confirmed, your GP or asthma nurse will start treatment under NG245 — usually a combination inhaler containing a low-dose steroid and formoterol, used when you have symptoms (AIR), or daily plus extra doses (MART) if your symptoms are frequent. A blue reliever on its own is no longer recommended. You should also get:
- a personalised written or digital asthma action plan
- inhaler technique training; see our guide to how to use an asthma inhaler
- a review a few weeks after starting treatment, then an asthma review at least once a year
- advice on your triggers; see our asthma triggers guide
Once you’re diagnosed and on stable treatment, Access Doctor can supply the same inhaler you already use to adults after an online consultation with a pharmacist independent prescriber, and we share it with your GP. Approved before 3pm, Monday to Friday? Dispatched the same day, by Royal Mail Tracked 48, Tracked 24 or Special Delivery guaranteed by 1pm next working day: you choose. Delivery is free on orders over £40.
Already Diagnosed? Order Your Usual Inhaler
Access Doctor is a GPhC-registered pharmacy (#9011198). If you’re an adult with diagnosed asthma or COPD, a pharmacist independent prescriber reviews your online consultation and supplies your usual inhaler if it’s safe, and we let your GP know. Approved before 3pm, Monday to Friday? Dispatched the same day, by Royal Mail Tracked 48, Tracked 24 or Special Delivery guaranteed by 1pm next working day: you choose. Delivery is free on orders over £40.
Start Asthma Consultation →Safety, your GP and free NHS options
Asthma inhalers are prescription-only medicines in the UK. Before anything is supplied, a GPhC-registered pharmacist independent prescriber reviews your consultation, including the other medicines you take — some, such as beta-blockers, can make asthma worse.
- Relievers on their own: since 2024, UK guidance (NICE, BTS and SIGN) says adults with asthma shouldn’t be treated with a reliever alone, and in April 2025 the MHRA reminded prescribers not to supply one without an inhaled steroid. If you don’t use a preventer, we can supply a reliever once and will ask you to see your GP or asthma nurse.
- How many: up to 2 reliever inhalers per order. If you’ve needed 3 or more in the last 12 months, we’ll ask you to have an asthma review first; needing that many is linked to a higher risk of severe attacks.
- Read the leaflet in the pack, and ask a pharmacist or asthma nurse to check your technique.
- Report side effects to a pharmacist or doctor. You can also report them to the MHRA through the Yellow Card scheme.
We share your consultation with your GP because asthma needs ongoing care. Access Doctor is a private service, and there are free NHS options:
- Your GP practice: asthma reviews (at least once a year) and repeat prescriptions; many practices let you order repeats through the NHS App.
- Prescription charges: NHS prescriptions are free in Scotland, Wales and Northern Ireland. In England a charge applies unless you’re exempt, and a prepayment certificate can save money if you need several items.
- Urgent advice: NHS 111 (NHS 24 on 111 in Scotland), or 999 in an emergency.
Frequently Asked Questions
How is asthma diagnosed in the UK?
Your GP or asthma nurse first asks about your symptoms and examines you. If asthma seems likely, the 2024 NICE, BTS and SIGN guideline (NG245) sets out tests for adults in this order: a blood eosinophil count or a FeNO breath test (asthma is diagnosed if eosinophils are raised or FeNO is 50 ppb or more); then spirometry with a reliever (an FEV1 rise of 12% and 200ml or more); then twice-daily peak flow readings for 2 weeks (20% variability or more); then a bronchial challenge test if needed. Tests only confirm asthma when your symptoms also fit.
Can asthma be diagnosed without spirometry?
Yes, in some cases. Under NG245, adults first have a blood eosinophil count or FeNO test, and if either is clearly raised and your symptoms fit, asthma can be diagnosed without spirometry. If those tests don’t confirm it, spirometry with a reliever is the next step, or twice-daily peak flow readings if spirometry isn’t available. A diagnosis based on symptoms alone, without any objective test, isn’t recommended because it leads to both over- and under-diagnosis.
What is a FeNO test for asthma?
FeNO stands for fractional exhaled nitric oxide. You breathe out steadily into a small machine for about 10 seconds, and it measures nitric oxide in your breath, which is raised when there’s a certain type of inflammation (eosinophilic or “type 2”) in the airways. Under NG245, a FeNO of 50 ppb or more in adults, or 35 ppb or more in children aged 5 to 16, supports asthma when the symptoms fit. It’s quick and painless, and is available in many GP practices and community diagnostic centres.
What peak flow variability indicates asthma?
Under NG245, you measure your peak flow twice a day (morning and evening) for 2 weeks. Variability of 20% or more supports asthma. It’s worked out each day as the difference between your highest and lowest reading, as a percentage of the average of the two, then averaged over the 2 weeks. Readings that are lowest in the morning are typical of asthma. Your GP or asthma nurse will calculate it from your diary.
Can adults develop asthma and be diagnosed for the first time?
Yes. Asthma can start at any age. In adults it’s often linked to work exposures (occupational asthma, common in bakers, hairdressers, paint sprayers and people who work with animals), hormonal changes, chest infections or new allergies. If you have wheeze, breathlessness, chest tightness or a cough that keeps coming back, see your GP for tests. If symptoms improve at weekends or on holiday, tell them, as this can point to work. See our asthma guide.
What conditions can be confused with asthma?
Several conditions cause similar symptoms: COPD (usually in smokers, with narrowing that doesn’t fully reverse), heart failure (breathlessness worse lying flat, ankle swelling), inducible laryngeal obstruction, also called vocal cord dysfunction (noisy breathing in, often triggered suddenly), dysfunctional breathing or hyperventilation, and bronchiectasis (a long-term cough with phlegm). This is why objective tests matter: symptoms alone can’t reliably tell them apart.
Can I get an asthma diagnosis online?
No. Asthma needs tests that can’t be done online, such as FeNO, blood tests and spirometry, so see your GP. Access Doctor’s online service is only for adults who already have a diagnosis and need their usual inhaler. If you have new symptoms, or think you might have asthma, book an appointment with your GP practice. See our asthma inhalers online if you’re already diagnosed.
Should I start an inhaler before my asthma tests?
Ideally, tests are done before treatment starts, because inhaled steroids can lower FeNO and eosinophils and make asthma harder to confirm. If you’re very breathless, though, you may need treatment straight away; your GP will decide. Don’t borrow someone else’s inhaler while waiting, and call 999 if you’re struggling to breathe.
For symptoms, triggers and treatment, see our asthma guide.
References
- NICE, BTS and SIGN. Objective tests for diagnosing asthma in adults, young people and children aged 5 to 16 (NG245 pathway). rightdecisions.scot.nhs.uk
- NICE, BTS and SIGN. Asthma: diagnosis, monitoring and chronic asthma management (NG245), November 2024. nice.org.uk
- NHS. Asthma. nhs.uk
- MHRA Drug Safety Update. Short-acting beta-2 agonists (SABA): reminder of the risks from overuse in asthma and changes in the SABA prescribing guidelines, April 2025. gov.uk
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.