Peak flow monitoring (Peak expiratory flow (PEF) monitoring)
Using a small handheld device to measure how fast you can blow air out, usually tracked over days or weeks to help spot and monitor asthma.
✓ Medically reviewed by a GMC-registered consultant · last reviewed September 2026 · next review September 2027 · how we review
In short
- Peak flow measures how fast you can blow out; tracked over time it shows how much your airways open and close.
- A single reading means little — the pattern and variability over days or weeks is what matters.
- It is mainly a home test: you record your own scores and your clinician reviews the trend, so accurate technique and honest recording are important.
- It supports diagnosis and monitoring but does not replace other tests, and a normal score on a good day does not mean asthma is well controlled.
A plain-English summary. The detail — including risks and recovery — is below.
At a glance
A general guide. Your specialist will give you advice for your situation.
Cheap, quick and easy to do yourself at home.
Young children or anyone who cannot reliably perform a hard, sharp blow may not give meaningful readings.
Three quick blows take a minute or two; you record the best score and carry on as normal.
A confirmed personal best and clear green, amber and red action-plan zones.
Three quick blows take a minute or two; you record the best score and carry on as normal.
You get used to the technique and routine. Early readings can be less reliable until your technique settles.
A pattern builds up showing your typical scores and how much they swing across the day and week.
Your clinician looks at the diary, works out variability and your personal best, and discusses what it means for...
What is peak flow monitoring?
Peak flow monitoring means regularly measuring how fast you can blow air out of your lungs, using a small handheld device called a peak flow meter. The fastest of three blows is your peak flow score.
One reading on its own means little. The value comes from tracking readings over time — often twice a day for a couple of weeks — to see how much they vary. Airways affected by asthma tend to narrow and open more than healthy ones, so the score swings up and down more.
Peak flow monitoring helps with diagnosing asthma, judging how well it is controlled, and spotting early warning of a flare-up before you feel very unwell. It does not diagnose asthma on its own, and a single normal reading does not mean your asthma is well controlled.
Types, options & approaches
There may be different ways to do this. The right approach depends on the clinical question and your circumstances.
Options at a glance
These are the main approaches described in this guide. The right option depends on the diagnosis, your goals and what your clinician thinks is safe.
Diagnostic (serial) peak flow monitoring
Readings recorded at least twice a day, usually for two to four weeks, to look for the day-to-day variability that can support an asthma diagnosis.
Ongoing monitoring
Regular readings used over the longer term to judge how well asthma is controlled and to guide treatment.
Action-plan monitoring (zones)
Readings compared against your personal best, often using green, amber and red zones in a written asthma action plan to tell you when to act.
Occupational peak flow monitoring
Readings kept at and away from work to help show whether something at work is triggering asthma symptoms.
Preparing for your test
- Get a peak flow meter — these are available on NHS prescription, and your clinician can show you the correct technique.
- Use the same meter each time and take it with you to appointments, as different meters can read slightly differently.
- Measure at the same times each day, usually morning and evening, and before taking your inhalers.
- Stand or sit upright the same way each time, take the deepest breath you can, seal your lips around the mouthpiece and blow as hard and fast as you can.
- Do three blows each time and record the highest of the three.
- Use a peak flow diary or app to note the date, time and score, plus any symptoms or reliever use.
- Ask what your 'personal best' is, as your scores are judged against this rather than a single fixed number.
What happens
You do the test yourself. Set the marker to zero, stand or sit upright, breathe in fully, seal your lips tightly around the mouthpiece and blow out as hard and fast as you can in one short, sharp breath. Repeat three times and write down the highest score.
Most people measure twice a day, in the morning and the evening, ideally before using their inhalers, because readings are naturally lower first thing. You keep this up for the period your clinician suggests — commonly two to four weeks for diagnosis — and bring the diary back so the pattern can be reviewed. Your clinician looks at how high your scores are and, importantly, how much they vary across the day and from day to day.
Is this test right for me?
A good consultation should explore whether it's the right choice for you now — including reasons to wait or consider something else.
May not be suitable if…
- Young children or anyone who cannot reliably perform a hard, sharp blow may not give meaningful readings.
- It is the wrong tool when a precise measure of airway narrowing is needed — spirometry is more accurate.
- It should not be used to assess a severe asthma attack instead of seeking urgent care.
- It is unreliable as a sole basis for diagnosis without technique checking and other tests.
Delay or rearrange if…
- You have not yet been shown the correct technique, as poor technique makes readings useless.
- You are acutely unwell and need urgent treatment rather than home monitoring.
- You cannot commit to recording readings consistently over the monitoring period.
- You do not yet have a reliable meter or a diary to record in.
Alternatives to discuss
- Spirometry for a more accurate, one-off measure of airway function.
- Bronchodilator reversibility testing to look for improvement after a reliever.
- FeNO testing to look at airway inflammation.
- A bronchial challenge test in specialist settings when other tests are inconclusive.
- Symptom-based review alone where regular blowing is not feasible.
Before you decide
Use this as a shared-decision checklist. The aim is not just “can this be done?”, but whether it is right for you, now, with the risks and alternatives clearly understood.
What matters most to me?
Think about symptoms, daily life, work, caring responsibilities, sport, fertility, travel, appearance and anxiety — the right choice depends on your priorities, not just the medical facts.
What are all my options?
Ask about waiting, monitoring, medicines, rehabilitation, a smaller or larger procedure, a different test, NHS referral, or a second opinion where that would help.
What would make me pause?
Active infection, pregnancy, unstable medical problems, smoking, medicines that increase bleeding, poor support at home, or feeling pressured are all reasons to slow down and get tailored advice.
What happens if I do nothing today?
For some problems, waiting is safe; for others, delay can make treatment harder. A good consultation should explain the trade-off in plain English.
Benefits
- Cheap, quick and easy to do yourself at home.
- Tracks changes over time that a one-off clinic test would miss.
- Can support an asthma diagnosis by showing variability in your airways.
- Helps judge whether treatment is keeping asthma under control.
- Used with an action plan, can give early warning of a flare-up so you act sooner.
- Can help link symptoms to work or other triggers.
Risks & complications
- Readings that are unreliable if the technique is not consistent — for example not blowing hard enough or not sealing the lips.
- Feeling briefly light-headed or coughing after blowing out hard.
- Forgetting readings or recording them inaccurately, which makes the pattern hard to interpret.
- False reassurance from a normal reading on a good day when asthma is not actually well controlled.
- Effort or technique differences making scores look worse or better than they are.
- Relying on peak flow alone and missing the need for other tests.
- A severe blow triggering bothersome coughing or wheeze in very twitchy airways.
- Spread of infection if a meter is shared between people without cleaning.
Peak flow is effort- and technique-dependent, so its value depends on doing it the same careful way each time and recording honestly. It is also less sensitive than spirometry. The biggest pitfall is treating a single good reading as proof your asthma is controlled. Ask your clinician to check your technique and to set your personal best and action-plan zones.
Published figures to discuss
Peak flow monitoring is a self-administered, very low-risk measurement, so adverse-event rates are not meaningful. The real limitations are accuracy and reliability: results depend heavily on consistent technique and honest, regular recording, and peak flow is less sensitive than spirometry. Both falsely low (poor effort) and falsely reassuring (a single good reading) results are possible, so the pattern over time, not a single number, is what counts.
| Figure | Reported range | How to interpret it | Source / confidence |
|---|---|---|---|
| Physical harm from monitoring | Very low | Peak flow is a forceful blow into a handheld meter; light-headedness or cough can occur. | Guide sourcesClinical context |
| Peak-flow variability supporting asthma | NICE uses variability over 20% as a positive objective test in the right pathway | Technique, effort and meter consistency matter. A diary is only useful if readings are done properly and at the agreed times. | NICE NG245 — Asthma: diagnosis, monitoring and management (recommendations)nice.org.ukPublished figure |
| False reassurance | Recognised | Normal peak flow does not exclude asthma, particularly if symptoms are intermittent or the patient is already using preventer treatment. | Guide sourcesClinical context |
| Missed deterioration | Clinically important | An action plan should state what number or percentage drop means increase treatment, seek same-day advice or call emergency services. | Guide sourcesClinical context |
These are literature figures, not a personalised prediction. Your own risks and likely benefits depend on your circumstances, your health, and how your care is carried out and followed up.
What happens afterwards
There is nothing to recover from. Peak flow monitoring is something you do as part of normal life. The 'result' is the pattern your clinician reviews once you have collected enough readings.
- Scores that are a little lower in the morning than the evening.
- Mild coughing or light-headedness straight after a hard blow.
- Some day-to-day variation, which is part of what the test is looking for.
- Needing a few days of practice before your technique is consistent.
Aftercare
- Keep using the same meter and bring it, and your diary, to appointments.
- Record readings honestly, including low ones and any reliever use.
- Ask your clinician to confirm your personal best and your action-plan zones.
- Keep taking your inhalers and medicines as prescribed.
- Clean the mouthpiece as advised and do not share your meter with others.
- Note anything that seems to trigger low readings, such as work, exercise or allergies.
- Ask what reading should prompt you to take action or seek help.
- A peak flow meter (ideally the same one each time)
- A peak flow diary or app
- A note of the best times to measure (usually morning and evening)
- Your usual inhalers and a list of medicines
- Your personal best score, if known
- A written asthma action plan, if you have one
- Questions about your zones and when to seek help
⚠ Get urgent help if…
- Peak flow dropping into the amber or red zone of your action plan — follow the plan and seek advice.
- Worsening breathlessness, wheeze or chest tightness not relieved by your reliever inhaler — seek urgent help.
- Needing your reliever inhaler far more often than usual, or it not lasting.
- Being too breathless to speak in full sentences, eat or sleep — this is an emergency, call 999.
- Lips or fingertips turning blue, or feeling drowsy or confused — call 999.
- A steady fall in your scores over several days even if you feel only slightly worse.
Who to contact: your clinician, clinic or test provider first (keep their number to hand). For urgent advice when you can't reach them, call NHS 111. In an emergency, call 999.
General guidance — it doesn't replace the specific advice your specialist gives you.
Results & realistic expectations
Your clinician interprets peak flow by looking at your scores against your personal best and at how much they vary. UK guidance regards peak flow variability of 20% or more (over a period of monitoring) as supporting a diagnosis of asthma. For ongoing care, scores are often grouped into zones — green (good control), amber (caution) and red (urgent) — based on your personal best.
A reassuring result is steady readings close to your personal best with little variability. Peak flow cannot, on its own, confirm or exclude asthma, cannot tell you why a reading is low, and a single good reading does not prove your asthma is controlled.
Peak flow readings reflect your airways at the moment of testing, so monitoring is an ongoing tool rather than a one-off result. Your personal best can change over months and years, especially in children who are still growing, so it may need updating. Your clinician will advise how long to monitor and when to repeat it.
Related tests, treatments or support
Peak flow monitoring is often combined with spirometry, bronchodilator reversibility testing and FeNO when diagnosing asthma, and with a written asthma action plan for ongoing control. Keeping a symptom and reliever-use record alongside your readings makes the pattern more useful.
Follow-up & long-term care
You bring your completed diary to a review, where your clinician assesses the pattern, sets or updates your personal best and zones, and adjusts treatment if needed. If readings are very low or falling, you may be advised to seek help sooner rather than waiting for the next appointment.
- Recheck and update your personal best periodically, especially for growing children.
- Replace the meter if it is damaged or readings seem inconsistent.
- Keep monitoring as advised during treatment changes or after a flare-up.
- Review and update your asthma action plan zones with your clinician.
Repeat, follow-on and what comes next
- Monitoring is often repeated or extended if the diary is incomplete or the pattern is unclear.
- Your personal best and action-plan zones may need revising over time, especially as children grow.
- A normal monitoring period does not rule out asthma and may prompt other tests.
Ask what happens if the result is unclear or needs repeating, and what is included if further tests or follow-up are needed.
What good aftercare looks like
- A confirmed personal best and clear green, amber and red action-plan zones.
- A check of your technique and a route to ask questions.
- Review of your diary alongside symptoms and other tests, not in isolation.
- Clear advice on which readings should prompt action or urgent help.
What affects the cost
Costs vary a great deal between people and providers, and we don't publish prices. What matters is understanding what drives the cost and making sure your quote is complete. The main things that affect it:
- Whether a meter is supplied or you already have one (meters are available on NHS prescription).
- Whether monitoring is part of a wider private respiratory assessment or consultation.
- Who reviews and interprets your diary, and whether a follow-up appointment is included.
- Whether a written asthma action plan is provided.
- The clinic or provider and its location.
- Whether a peak flow meter and diary or app are included.
- Whether teaching of technique and setting of your personal best are included.
- Who will review the readings and explain the pattern.
- Whether a written asthma action plan is provided.
- Whether follow-up review of the diary is included or charged separately.
- The cancellation and rescheduling policy.
On the NHS? Peak flow meters are available on NHS prescription and monitoring is usually done at home; private respiratory assessments may include peak flow as part of a wider package.
You're entitled to your total cost in writing — including reports, follow-up and what happens if the result is inconclusive — before you decide.
Consent traps and marketing red flags
These are not small details. They are often where patients lose choice, time to reflect, or realistic expectations.
Consent traps
- No check that you can perform the technique correctly.
- Treating a single good reading as proof of good control.
- No personal best or action-plan zones set, so readings cannot be acted on.
- Relying on peak flow alone for diagnosis without other tests or symptom review.
Marketing red flags
- Claims that a meter or app alone can diagnose or manage asthma without clinical input.
- Selling devices as a complete substitute for proper assessment and an action plan.
- Implying a normal reading proves your lungs are healthy.
- Promising precise lung-function measurement that only spirometry can give.
Choosing a specialist safely
- Check the specialist is on the GMC Specialist Register for this area.
- Make sure they work at a CQC-registered service, and look for membership of the relevant Royal College or professional body.
- You're entitled to time to consider and to have your questions answered before you agree — the specialist who looks after you should explain it, not a salesperson.
- Be wary of pressure: time-limited offers or deposits taken before you've had time to think are red flags, not bargains.
- You're entitled to your total cost in writing — including any follow-up — before you decide.
Questions to ask your medical professional
Take this to your consultation. A good specialist will welcome every one of these.
- Is my technique correct, and can you watch me do it?
- What is my personal best, and how were my action-plan zones set?
- How long should I monitor, and how often each day?
- What level of variability or fall in my score should prompt me to act or seek help?
- How does my peak flow pattern fit with my other tests and symptoms?
- Are you on the GMC Specialist Register for this area, and which Royal College or professional body are you a member of?
- Will you be the specialist who carries out my test, and who looks after me afterwards?
- What are the risks for someone like me, and how often do your own patients have a problem or need it repeated or redone?
- What does a realistic result look like — and what can this test not achieve?
- What are my options, including waiting, doing nothing for now, or choosing a different approach?
- Can I have written information, results and aftercare instructions in a format I can use, including any accessibility or communication support I need?
- What is the total cost in writing, including any follow-ups, and how much time do I have to decide?
Frequently asked questions
Can I get a peak flow meter on the NHS?
How often should I measure my peak flow?
What is my 'personal best'?
Why are my morning readings lower?
Does a normal reading mean my asthma is fine?
Can peak flow alone diagnose asthma?
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How we made this page
Medically reviewed by a GMC-registered consultant. Written in plain English, checked against NHS, NICE, GMC and relevant Royal College / specialist-society guidance, and kept under review. No clinic paid to appear on this page, and we publish no pricing. This is general information to help you prepare — it is not a substitute for advice from your own clinician. How we review our guides →
Source hierarchy: UK regulator and NHS/NICE guidance first, then relevant Royal College or specialist-society guidance, then peer-reviewed evidence for procedure-specific figures where available.
Sources & standards: NHS — Peak flow test Asthma + Lung UK — Peak flow NICE NG245 — Asthma: diagnosis, monitoring and management (recommendations) NICE MIB282 — Smart Peak Flow for monitoring asthma
Reviews reflect patients' experience of care, not clinical outcomes. For procedure volumes and outcome data see PHIN.
Last medically reviewed 2026-09-21. Spotted something wrong or out of date? Report an error in this guide.
Related guides: FeNO test (exhaled nitric oxide) · Reversibility testing · Bronchial challenge (provocation) test · Body plethysmography (lung volume test) · Bronchial and transbronchial biopsy