Blood test
Arterial Blood Gas
An arterial blood gas (ABG) measures oxygen, carbon dioxide and acidity in blood taken from an artery, used to assess breathing and metabolic problems.
Quick answer
Arterial Blood Gas: what it is and what the results mean
An ABG is a blood test taken from an artery (usually at the wrist) rather than a vein. It measures oxygen and carbon dioxide levels, the blood’s acidity (pH) and related values, giving a detailed picture of lung and metabolic function.
- Why it is done: It is used in seriously unwell or breathless people to assess how well the lungs are moving oxygen and removing carbon dioxide, to check the body’s acid-base balance, and to guide oxygen and ventilation treatment.
- Understanding results: Results show oxygen and carbon dioxide levels and whether the blood is too acidic or alkaline, and why.
What it is
An ABG is a blood test taken from an artery (usually at the wrist) rather than a vein. It measures oxygen and carbon dioxide levels, the blood’s acidity (pH) and related values, giving a detailed picture of lung and metabolic function.
Why it is done
It is used in seriously unwell or breathless people to assess how well the lungs are moving oxygen and removing carbon dioxide, to check the body’s acid-base balance, and to guide oxygen and ventilation treatment.
What to expect
A sample is taken from an artery, most often at the wrist, using a small needle. It is briefly more painful than a normal blood test, and pressure is applied afterwards. Results are usually available within minutes.
Understanding the results
Results show oxygen and carbon dioxide levels and whether the blood is too acidic or alkaline, and why. This guides urgent decisions about oxygen, breathing support and treatment of the underlying cause.
Good to know
Risks and limitations
It is more uncomfortable than a venous sample and carries a small risk of bruising or, rarely, damage to the artery. It gives a snapshot in time, so it may be repeated to track changes.
Education and reference only. This explains the test in general terms and is not medical advice. Always follow the specific instructions from the team arranging your test, and discuss your results with your clinician.
Answers
Arterial Blood Gas: frequently asked questions
Why is blood taken from an artery instead of a vein?
Arterial blood gives an accurate measure of oxygen and carbon dioxide as the blood leaves the lungs, which venous blood cannot provide. This is essential for assessing breathing and acid-base balance.
Is an arterial blood gas painful?
It is usually more uncomfortable than a normal blood test because arteries are deeper and more sensitive, but it is quick. Local anaesthetic is sometimes used, and pressure is applied afterwards.
Related tests
Other blood test investigations
Sources
Where this is drawn from
- NHS — Tests and treatments
- NICE — diagnostic guidance
- Association for Laboratory Medicine / relevant professional body
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