Capillary blood gas (CBG) test

A capillary blood gases test will determine how efficiently your lungs are working

What is a blood gas test?

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

By measuring the levels of these two gases in your blood your consultant can tell how efficiently your lungs are working, particularly if you are suffering from chronic obstructive pulmonary disease (COPD).

Need to know

What happens during the blood gas test?

Your tester will apply a special cream to 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 chronic obstructive pulmonary disease (COPD) symptoms.

How to prepare

You should avoid wearing any earrings to your test. Other than that, there are no specific preparations that need to be made.

After the test

Your consultant will discuss your tests results with you. The results will be a set of readings in relation to oxygen, carbon dioxide and acidity levels in your blood. Abnormal levels 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. You'll be able to return to normal activities immediately after the test.