At a Glance
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).
The Test Sample
What is being tested?
- 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 acid-base balance. 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 anticoagulants 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.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.