Summary

• Blood gas testing shows if you have the right amount of oxygen and carbon dioxide in your blood. 
• It also shows whether your blood is too acid or too alkaline. This is called your acid-base balance or your pH level.
• In blood gas testing, samples of your blood are usually taken from an artery.
• Sometimes this can be difficult and so in certain circumstances blood can be collected from a vein. This is called venous blood gas testing.
• Venous blood gas testing is more comfortable to have, and it is often preferred in hospital emergency departments and ICU because it is easier to collect a blood sample from a vein and has a lower risk of complications.
• Although it is less accurate at measuring oxygen levels, venous blood gas testing can provide useful information about conditions where this is not the main reason for testing.

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:

• problems with your lungs and breathing such as in chronic obstructive pulmonary disease (COPD),
• asthma,
• kidney disorders,
• uncontrolled diabetes,
• shock,
• sepsis, and 
• heart disease.

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?

• Your lungs bring oxygen into your body.  As air is inhaled into your lungs it passes down your airways to tiny sacs called air sacs where it is absorbed into the surrounding blood vessels.
• Once taken up by your blood, oxygen sticks to haemoglobin, the red substance in your red blood cells.
• Oxygen is transported all over your body where it is used by your cells to generate energy.
• In order to produce energy, oxygen binds with glucose molecules which have been processed from the food you eat. This releases water and bicarbonate, a waste product.
• Bicarbonate passes from the cells into the blood where it attaches to haemoglobin and is transported back to your lungs.
• In your lungs, bicarbonate is separated from the haemoglobin and is breathed out as carbon dioxide. 

The blood gas test measures these components of your blood.

• pO₂ measures the pressure of oxygen dissolved in your blood. It shows how well oxygen is moving from your lungs into the blood.
• pCO₂ measures the pressure of carbon dioxide dissolved in your blood. It shows how well carbon dioxide is moving out of the body.
• Oxygen saturation measures the amount of oxygen being carried by the haemoglobin in the red blood cells.
• pH shows if you blood is too acidic or too alkaline. A pH of less than 7.0 is acidic, and a pH greater than 7.0 is alkaline – called basic. A low pH level suggests your blood is more acidic and has a high carbon dioxide level. A high pH level suggests your blood is more alkaline and has a higher bicarbonate level.
• Bicarbonate in your body helps control your pH level and keeps your blood from becoming too acidic or too alkaline.

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 take 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 an emergency when a quick assessment is needed, and an arterial sample (a sample from your artery) will take too long to collect. 
• If you have poor blood circulation.
• To avoid the discomfort from an arterial blood collection especially in children. 
• To monitor chronic respiratory or metabolic conditions .
• As an initial assessment of acid-base balance before confirming with a blood sample from an artery, if required.

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).

• Reference intervals are the range of results expected in healthy people.
• When compared against them your results may be flagged high or low if they sit outside this range.
• Many reference intervals vary between labs so only those that are standardised or common across most laboratories are given on this website.

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.

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:

• assess acid-base balance,
• check for low blood glucose levels,
• monitor suspected respiratory issues, after an initial arterial blood gas test has been done,
• assess electrolyte disturbances that can impact the baby's health, or
• monitor babies suspected of having sepsis.

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:

• Why does this test need to be done?
• Do I need to prepare (such as fast or avoid medications) for the sample collection?
• Will an abnormal result mean I need further tests?
• How could it change the course of my care?
• What will happen next, after the test?

More information

Pathology and diagnostic imaging reports can be added to your My Health Record.

You and your healthcare provider can now access your results whenever and wherever needed. Get further trustworthy health information and advice from healthdirect.