At a glance
Also known as
Arterial blood gases; ABGs; blood gas analysis
Why get tested?
To determine if you have an imbalance in the amount of oxygen gas (O2) and carbon dioxide gas (CO2) in your blood or an acid-base imbalance (i.e. if your blood is too acidic or too alkaline). These problems may indicate a respiratory (lung / breathing) or kidney disorder or some other disorder of metabolism.
When to get tested?
If your doctor suspects that you have symptoms of an oxygen/carbon dioxide imbalance or an acid-base imbalance. These include difficulty breathing, shortness of breath, or rapid breathing. You may also be tested to monitor the effectiveness of oxygen therapy (used when you have a condition that causes an oxygen shortage) and during operations to monitor your blood's oxygen and carbon dioxide levels.
A blood sample collected from an artery, usually the radial artery in the wrist (located on the inside of the wrist, below the thumb, where you can feel your pulse). Sometimes blood gas testing is done on blood taken from a vein or a capillary (done by heel prick in babies).
What is being tested?
Blood gas tests tell your doctor if you have enough oxygen in your blood and whether your blood is too acidic () or too alkaline (). Blood gas tests directly measure:
- pH – a measure of the level of hydrogen ion (H+), which indicates the acid-base status of your blood. The pH of your blood decreases (becomes more acidic) with increased amounts of CO2 and other acids, and the pH increases (blood becomes more alkaline) with decreased CO2 or increased amounts of bases like bicarbonate (HCO3-).
- PO2 – the partial pressure of O2 (the amount of oxygen gas dissolved in blood).
- PCO2 – the partial pressure of CO2 (the amount of carbon dioxide gas dissolved in the blood). As PCO2 levels rise, blood pH levels will decrease, becoming more acidic. As PCO2 decreases, pH levels will rise, making the blood more alkaline.
Calculations or measurements also can be done to give other values, such as:
- O2 saturation – an indication of how much of the haemoglobin, the oxygen-binding substance in the blood, is actually carrying oxygen. This result may be an estimate, calculated from other tests or, if more advanced equipment is available in the laboratory, it may be accurately measured.
- HCO3- (bicarbonate) – the main form of CO2 in the body, it can be calculated from the pH and PCO2. It is a measurement of the metabolic component of the . HCO3- is excreted and reabsorbed by the kidneys in response to pH imbalances and is directly related to the pH level; as the amount of HCO3- rises, so does the pH.
- Base excess/base deficit - a calculated number that represents a sum total of the metabolic buffering agents (anions – negatively charged ions) in the blood; these anions include haemoglobin, proteins, phosphates, and HCO3- (bicarbonate, which is the dominant anion). The anions try to compensate for imbalances in the blood pH. The doctor will look at the HCO3- and base excess/deficit results to evaluate the total buffering capacity when deciding on a treatment to correct an imbalance.
How is the sample collected for testing?
Blood gas analysis has traditionally meant a test of arterial blood, rather than of venous blood, which is used for almost all other blood tests. This is because arterial blood carries oxygen to the body while venous blood carries waste products to the lungs, so the gas and pH levels of arterial and venous blood are somewhat different, particularly the oxygen level. However, in some cases, blood gas analysis of venous blood is useful and in babies capillary blood from heel pricks is used.
An arterial blood sample is usually collected from the radial artery in the wrist (located on the inside of the wrist below the thumb, where you can feel your pulse). Blood can also be collected from the brachial artery in your elbow or the femoral artery in your groin.
In newborns who experience difficulty in breathing right after birth, blood may be collected from the vessels of the umbilical cord.
If you are on oxygen therapy, the O2 will either be turned off for 20 to 30 minutes before the collection for a 'Room Air' test or, if you cannot tolerate this or if the doctor wants to check your oxygen levels with the O2 on, the amount of oxygen you are breathing will be recorded.
After the arterial blood has been collected you will be instructed to hold the site firmly for at least 5 minutes. Since blood pumps through the artery, the puncture will take a while to stop bleeding. If you are taking or aspirin, it may take up to 10 or 15 minutes to stop. The doctor will verify that the bleeding has stopped and put a wrap around your wrist, which should be left in place for an hour or so.
Can this test be done in a doctor's office?
Blood gas measurements are usually done in a hospital since the analysis should be done immediately after the blood has been collected and requires specialised equipment. Most GPs do not have such equipment in their surgeries.
I've had pneumonia before and currently have asthma. Why has my doctor never requested this test on me?
Most cases of pneumonia or can be diagnosed by symptoms and monitored by listening to your chest sounds or by examining the results of spirometry tests or chest x-rays. Most of the time, asthma will respond to your usual medications and pneumonia to antibiotics. Blood gas tests may be necessary if you have severe, sudden or long-term breathing problems. In these cases, blood gas tests are usually done in a hospital setting.
Is there any other way to measure my oxygen levels?
A pulse oximeter is a device used to continuously monitoring oxygen saturation without the use of a needle to obtain a blood sample. A small clip-like device called a sensor is attached to the end of the finger or earlobe. The sensor reads light that is transmitted through the skin. Pulse oximeters are useful for monitoring oxygen saturation, but their accuracy can be affected by the presence of abnormal forms of haemoglobin, like carboxyhaemoglobin (present in carbon monoxide poisoning), low pulse rate due to poor circulation, and very low levels of haemoglobin due to severe anaemia
. Some coloured nail polishes may interfere with devices that clip on the finger. While pulse oximeters can be useful, they do not give any information about the patient’s acid-base status.
Can I have a venous blood gas test instead of an arterial blood gas test?
Depending on your medical problem, it may be possible to have a venous blood gas test, but if your doctor wants to check your oxygen levels, it is necessary to have an arterial blood gas test.