Breathing and lung function tests

Blood gas test

What is it?

A blood gas test is used to measure how much oxygen and carbon dioxide are in your blood. The test is called an arterial blood gas test if the sample is taken from your wrist. It’s called a capillary blood gas test if the sample is collected from your earlobe.

What’s it used for?

It is used to check how well your lungs are working and whether they’re able to exchange oxygen and carbon dioxide efficiently. It can be used to see if you need oxygen therapy.

What happens during the test?

The tester will take a small sample of your blood. They will normally take this by using a needle and a syringe in one of the arteries of your wrist (or occasionally from the inside of your elbow). Sometimes some local anaesthetic is applied.

Some hospitals check blood gas by taking a blood sample from your earlobe. Your tester will put a special cream on your earlobe that helps increase blood flow. This makes your ear go red and feel hot. The blood vessels in your ear lobe will then contain about the same amount of oxygen as blood taken from your artery. After a few minutes the tester can take a sample by making a tiny cut and catching the blood droplet that forms.

The earlobe method can’t usually be used if you need to have the test when you are unwell (such as when you’re admitted to hospital with a flare-up of COPD symptoms).

What will the results look like?

The results will be a set of readings in relation to oxygen, carbon dioxide and acidity. Abnormal results of any of the gas components may mean your body is not getting enough oxygen or is not getting rid of enough carbon dioxide.

A high level of carbon dioxide may mean that your breathing is shallow at night and you may benefit from using a ventilator device at home

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Last medically reviewed: January 2017. Due for review: January 2020

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.