Peak flow is a measurement of how quickly you can blow air out of your lungs. It's used to help diagnose and monitor asthma.
The test involves blowing as hard as you can into a small handheld device called a peak flow meter.
These are available on prescription or you can buy them from most pharmacies.
Measuring peak flow
By measuring how fast you're able to breathe out, your peak flow score can show if your airways are narrowed.
This could be a sign that you have asthma. But other tests such as spirometry may be needed to confirm the diagnosis.
Checking your peak flow regularly can be a useful way of monitoring your asthma.
It can show if:
- your asthma is getting worse
- your medicine is working
- you're having an asthma attack
Measuring your peak flow before and after exposure to a possible asthma trigger may also show what is causing your symptoms.
How to measure your peak flow
Your GP or nurse will show you how to measure your peak flow to monitor your asthma at home.
To measure your peak flow:
- find a comfortable position, either sitting or standing
- reset your peak flow meter so the pointer is pushed back to the first line of the scale – this is usually 60
- hold the peak flow meter so it's horizontal and make sure that your fingers are not obstructing the measurement scale
- breathe in as deeply as you can and place your lips tightly around the mouthpiece
- breathe out as quickly and as hard as you can
- when you've finished breathing out, make a note of your reading
This should be repeated 3 times. The highest of the 3 measurements should be recorded as your peak flow score.
Use a diary or chart to record your score.
Your peak flow score
Your peak flow score will be displayed on the side of your peak flow meter. This is given in litres of air breathed out per minute (l/min).
What's considered a normal score depends on your age, height and gender.
A big difference between your current and best score could be a sign that:
- your condition is becoming poorly controlled
- that you're having an asthma attack
What to do if your peak flow is low
You may just need to use one of your inhalers, or you may need to get medical help. See what to do if you have an asthma attack for more information.
Non-urgent advice: Speak to your GP or practice nurse if:
- you're concerned about your peak flow score and do not know what to do
- you've been diagnosed with asthma and do not have a personal action plan