Summary
What is blood gas testing?
Blood gas testing measures the amount of oxygen and carbon dioxide in your blood. It also assesses the acidity of your blood – your acid-base balance or pH level.
You could have blood gas tests to investigate any one or more of a wide range of health conditions. These include:
You may also have these tests to find out whether any treatment you have been having is working.
Blood gas testing is commonly used in the hospital emergency department, respiratory clinic, during surgical procedures, in intensive care and in the ambulance to quickly check a critical patient’s lung function during transport. The emergency team will give any treatment needed while investigating the cause of the problem.
Oxygen is breathed in and carbon dioxide is breathed out – what happens in between?
The blood gas test measures these components of your blood.
Your pH or acid-base balance
Your kidneys help keep acidity levels stable when they filter your blood by removing or reabsorbing bicarbonate.
The less bicarbonate you have in your blood, the more acidic it becomes. The more bicarbonate you have, the more alkaline your blood becomes.
A blood sample from an artery or vein?
Most routine blood tests require a sample from a vein in your arm. In blood gas testing, the sample of your blood is best taken from an artery, usually in your wrist.
This is because arteries carry blood away from the heart, and veins carry blood towards the heart. The oxygen in the blood in your arteries has not been used by your tissues and organs. Blood from the arteries is more useful in showing if there is a problem, especially when levels of oxygen are being measured.
However, a blood sample can be difficult to take from an artery. This can be due to practical reasons such as if you have poor circulation or low blood pressure. Also, having a sample of blood taken from an artery can be more uncomfortable. This is because arteries are located more deeply than veins and they have thicker walls to penetrate. Taking blood from an artery also carries more risks such as a haematoma (blood leaking into your tissue) and infection.
Venous blood gas testing
Depending on your medical problem, it may be possible to have a venous blood gas test when the main purpose of the test is to assess your acid-base balance, metabolic condition, and/or measure carbon dioxide levels.
Venous blood gas tests are becoming more widely used as an alternative to arterial blood gas tests in certain situations, for example:
In the intensive care unit (ICU) or emergency department (ED) venous blood samples are sometimes preferred over arterial samples because they are more convenient to take when a patient has a central venous catheter, a small tube inserted into the vein which allows the medical team to take blood samples without using a needle, in place.
Sometimes, venous blood gas tests are used in combination with pulse oximetry – this is with a small device (sensor) usually clipped to your finger, toe or earlobe in hospital. The sensor reads light that is transmitted through the skin to measure the amount of oxygen in your blood. Pulse oximeters are useful for monitoring the amount of ocygen in your blood, but their accuracy can be affected by some blood conditions, low pulse rate due to poor circulation, and very low levels of haemoglobin due to severe anaemia.
If your medical team is mostly concerned about your oxygen levels, they may not be able to substitute a sample of blood from your vein with a sample from your artery. A blood sample from your vein is not useful in assessing blood oxygen levels because oxygen has already been taken up by your body’s tissues by the time the blood reaches your veins.
Why get tested?
Blood gas tests are ordered when you have difficulty breathing or shortness of breath or an acid-base imbalance. Many conditions can cause imbalances and while blood gas tests do not show the direct cause of your imbalance, they will point to either a respiratory (lungs), kidney or metabolic (diabetes, liver, pancreas) problem.
For more on how your body uses oxygen and how your lungs and kidneys work to help keep your acid-base balance normal see blood gases arterial.
Having the test
Sample
Blood from a vein. The sample must reach the laboratory within 30 minutes of collection.
Preparation
None.
Your results
Reference intervals
Your results will be compared to reference intervals (sometimes called a normal range).
If your results are flagged as high or low this does not necessarily mean that anything is wrong. It depends on your personal situation. Your results need to be interpreted by your doctor.
Adult venous blood gas reference intervals. | |
acid-base balance: | 7.32 to 7.43 |
pCO2 (carbon dioxide): | 37 to 50 mmHg |
pO2 (oxygen): | 36 to 44 mmHg |
Bicarbonate: | 22 to 28 mmol/L |
O2 (oxygen) saturation: | 70 to 80% |
Modern blood gas instruments also measure several other chemicals in your blood such as sodium, potassium, chloride, glucose, lactate, ionised calcium and haemoglobin levels. |
Any more to know?
Venous blood gas testing is also used to check newborns especially when arterial sampling is not possible or to avoid repeated arterial sampling which can be painful for the baby. This can be done after an initial arterial blood gas test has been performed. It is used to:
Questions to ask your doctor
The choice of tests your doctor makes will be based on your medical history and symptoms. It is important that you tell them everything you think might help.
You play a central role in making sure your test results are accurate. Do everything you can to make sure the information you provide is correct and follow instructions closely.
Talk to your doctor about any medications you are taking. Find out if you need to fast or stop any particular foods or supplements. These may affect your results. Ask:
More information
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