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.
• When oxygen levels are being measured, blood gas samples are usually taken from an artery.
• In some cases, blood collected from a vein might be used, but this is less useful in measuring the level of oxygen in your blood.

What is blood gas testing?

Blood gases 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),
• kidney disorders,
• uncontrolled diabetes, and 
• heart disease.

You may also have these test 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. Your emergency team will give you any treatment needed to regain your body’s normal balance while investigating what is causing the problem.

• The blood oxygen measurement shows how well your lungs bring oxygen from the air into your blood when you breath in.
• The carbon dioxide measurement shows how well your lungs remove carbon dioxide from your blood when you breath out.
• The more carbon dioxide you have in your blood the more acidic it becomes. The less carbon dioxide you have, the more alkaline your blood becomes. Your kidneys help keep this in balance when they filter your blood by removing and reabsorbing carbon dioxide (in the form of bicarbonate).

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. 

Why get tested?

Blood gas tests are ordered when you have difficulty breathing or shortness of breath or a 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.

Between them your lungs and your kidneys help keep the pH balance normal and your body will try to restore any imbalance by itself.  

Having the test

Sample

Most of the routine blood tests take a sample from a vein. In this test, when oxygen needs to be measured, a sample of your blood is usually taken from an artery 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 haematoma (blood leaking into your tissue) and infection, and although rare, blood clotting, which can result in peripheral nerve damage.

Depending on your medical problem, it may be possible to have a venous blood gas test, but if your medical team wants to check your oxygen levels, it is important to have an arterial blood gas test.

Preparation

None.

Your results

Abnormal results of any of the blood gas components may mean that your body is not getting enough oxygen, is not removing enough carbon dioxide, or that there is a problem with the acid and base (alkaline) balance in the blood. 

If left untreated, these conditions could eventually become life threatening. The severity of the problem depends on whether the cause is acute (sudden onset and short term), or chronic (long-term), and whether your body is exhausted from trying to compensate for the imbalance.

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.

The reference intervals or normal ranges of arterial blood are:

• pO₂: 75 to 105 mm Hg.
• pCO₂:  35 to 45 mm Hg.
• Oxygen saturation: 94 to 100 per cent.
• pH: 7.35 to 7.45.
• Bicarbonate: 22 to 26 mmol/L.

More to know?

Blood gas testing is also used to check newborns who are having difficulty breathing, especially babies in neonatal intensive care. It is important to make sure that the baby is getting enough oxygen, and their body is maintaining the right acid-base balance. Often testing will be repeated. Blood samples can be collected from an artery, vein, or the umbilical cord. 

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.