At a Glance
Why Get Tested?
To determine if you have an imbalance in the amount of oxygen gas (O2) or carbon dioxide gas (CO2) in your blood or an acid-base imbalance (i.e. if your blood is too acidic/ alkaline), which may indicate a respiratory (lung/breathing), metabolic, or kidney disorder
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 (hyperventilation). 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. If you have chronic obstructive pulmonary disease (COPD) this test may be used to assess if you need long term oxygen therapy.
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). A capillary blood from a heel-prick may be used for babies
Test Preparation Needed?
Typically, none. However, if you are on oxygen therapy, the O2 may either be turned off for 20 to 30 minutes before the collection for a "Room Air" test or, if this cannot be tolerated or if the doctor wants to check your oxygen levels with the O2 on, the amount of oxygen being taken will be recorded.
The Test Sample
What is being tested?
Blood gas tests tell your doctor if you have enough oxygen in your blood and whether or not your blood pH is balanced - not too acidic (acidosis) or too alkaline/basic (alkalosis). Blood gas tests directly measure:
- pH – a measure of the level of hydrogen ion (H+), which indicates the acid/alkali 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). Decreased PO2 may result in an elevated PCO2 and decreased pH
- 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 – a calculation of how much oxygen is bound to haemoglobin in the red blood cells and available to be carried through the arteries to nourish the body’s cells
- HCO3-; (bicarbonate) 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
How is the sample collected for testing?
Since arterial blood carries oxygen to the body and venous blood carries waste products to the lungs, the gas and pH levels will not be the same in both. Arterial blood is almost always used for blood gas analysis, but in some cases, such as for babies, whole blood from heel pricks may be 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). A circulation test called an Allen test will be done before the collection to make sure that you have adequate circulation in your wrist. The test involves compressing both the radial and the ulnar wrist arteries, then releasing each in turn to watch for "flushing" (the return of blood to your hand). If your hand does not flush, your other wrist will be tested. 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 both the umbilical artery and vein and tested separately.
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 taking 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 awhile to stop bleeding. If you are taking blood thinners or aspirin, it may take up to ten or fifteen 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.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Typically, no test preparation is needed. However, if you are on oxygen therapy, the oxygen may either be turned off for 20 to 30 minutes before the collection for a "Room Air" test or, if this cannot be tolerated or if the doctor wants to check your oxygen levels with the oxygen on, the amount of oxygen being taking will be recorded. This is usually expressed as fraction of inspired oxygen in percent (e.g., 30% FIO2) or as liters of O2 flowing per minute.
Ask a Laboratory Scientist
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.