Having a capillary blood gas test

What is a CBG test?

A CBG test is a type of blood test which is used to look at the oxygen and carbon dioxide levels in your blood. The level of these gases in your blood is controlled by your lungs and are important for controlling your breathing

A CBG test will also measure your acid-base balance (how acidic or alkaline your blood is). This needs to be at the correct balance to ensure that your body is functioning properly. If your blood oxygen or carbon dioxide levels go outside of the normal range, it can be dangerous, as it can put undue stress on your organs including the heart and kidneys.

The blood sample is taken from your earlobe from a small blood vessel called a capillary. Blood is taken from the earlobe because it’s easy to access your capillaries and is generally most comfortable.

How is a CBG performed?

To collect the blood sample, your physiologist will warm up your earlobe to increase the blood flow. They may use warm water or rub your earlobe with a piece of gauze to warm up your ear. Once your ear is warm enough, the physiologist will wipe your earlobe clean with an alcohol wipe - this may feel slightly cool on your skin.

Next, they will place a bung behind your ear and make a small scratch on your earlobe. This may sting for about a second. After wiping the earlobe again, the physiologist will collect the blood sample into a capillary tube. Once the sample has been collected, gauze will be held on your earlobe to help stop the bleeding.

Before your test

As the sample will be collected from your earlobe, please remove any earrings beforehand so that your physiologist can easily access your earlobe. If you have long hair or hair that falls in front of your ears, please either tie your hair up or clip it to the side. It may be useful to bring a hairband or hair clips with you to the appointment.

Please take all your regular medications as prescribed on the day of your test. Your physiologist will check if you are currently on any medication, such as blood thinning medication, before the test so that any necessary adjustments to the way we carry out the test can be made.

If you use additional oxygen, your physiologist may ask you to take this off for at least 20 minutes prior to taking the blood sample. This will depend on what your doctor has requested. If you are unsure, please ask your physiologist or contact us.

After your test

Your physiologist will ensure that your ear has stopped bleeding and will place a plaster on your ear. If your ear continues to bleed, the physiologist will place a wound dressing patch on your ear before placing the plaster on it. Please leave the plaster on your ear until the following morning, as there is a small chance that it could continue bleeding when your ear rubs against your pillow whilst you sleep.

Your CBG test results

The physiologist will send your test results to the doctor who referred you for the CBG. This will be uploaded onto your electronic patient record. If required, the doctor will organise a follow up appointment to discuss the results with you.

Risks and side-effects

Serious complications are rare. Risks are kept to a minimum by your physiologist. Potential risks for this test include:

• Dizziness and fainting
• Bruising
• Continuous bleeding
• Rebleeding if something rubs against the site
• Torn earlobe.

CBG test alternatives

The alternative to a CBG test is an arterial blood gas test. A doctor is required to perform an arterial blood gas. This measures all the same parameters as a capillary blood gas test; however, the blood sample is taken from an artery in the wrist. This is a more accurate way of measuring blood gases but tends to be a lot more painful.