Peak flow is a simple measurement of how quickly you can blow air out of your lungs. It's often used to help diagnose and monitor asthma.
A peak flow test involves blowing as hard as you can into a small, hand-held device called a peak flow meter.
These are available on prescription or can be bought from most pharmacies.
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By measuring how fast you're able to breathe out, your peak flow score can indicate whether your airways are narrowed.
If you've already been diagnosed with asthma, measuring your peak flow regularly can be a useful way of monitoring your condition.
Your score can show whether or not your condition is getting worse. It can also check if your medication is working, and indicate whether you're having an asthma attack.
Measuring your peak flow before and after exposure to a possible asthma trigger, such as something you're allergic to or a substance you're exposed to at work, may also show if anything in particular causes your symptoms.
The first time your peak flow is measured, you'll be taught how to do it by a doctor or nurse. Thereafter, they may advise you to carry out the test regularly at home using your own peak flow meter.
To measure your peak flow:
This should be repeated three times, and the highest of the three measurements should be recorded as your peak flow score.
If you're monitoring your asthma at home, you may have a diary or chart to record your score. You can download a peak flow diary (PDF, 2.2Mb) from the Asthma UK website if you don't have one.
Your peak flow score – also known as your peak expiratory flow (PEF) – 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.
To help diagnose asthma, your result can be compared to what would usually be expected for someone of your age, height and gender.
A significant difference between your score and a normal score, or a difference in your scores in the morning and evening or when your symptoms are good and bad, may suggest you have asthma.
If you're monitoring your asthma at home, your score should be compared to your best result at a point when your condition was well controlled.
A big difference between your current and best score could be a sign that your condition is becoming poorly controlled or that you're having an asthma attack.
If you have a personal asthma action plan, it might say what you should do when your peak flow score falls to a certain level.
Speak to your GP or asthma nurse if you've been diagnosed with asthma and don't have a personal action plan, or if you're concerned about your peak flow score and don't know what to do.
You can download a personal action plan (PDF, 681kb) that you can fill in with your doctor or nurse from the Asthma UK website.