What is an arterial blood gas test?

This page was reviewed under our medical and editorial policy by

Peter Baik, DO, Thoracic Surgeon

This page was updated on January 8, 2023.

An arterial blood gas test (ABG) is a diagnostic tool used to evaluate the blood’s levels of oxygen and carbon dioxide. It helps determine lung function by measuring how effectively the lungs move oxygen into the blood and carbon dioxide out of the blood. The test may also help detect a pH imbalance—if it’s too acidic or too alkaline—that may point toward a respiratory disorder.

How is an arterial blood gas test performed?

An ABG is usually performed in a hospital, laboratory or surgical center. It’s done using arterial blood, or blood drawn directly from an artery (before the blood has passed through the body’s tissues and oxygen has been used up). The sample of blood is usually taken from the radial artery of the wrist below the thumb, or sometimes from the brachial artery near the elbow or the femoral artery in the groin. Before the test, doctors will make sure there’s enough circulation in the area by compressing nearby blood vessels and watching for a “flush” showing that blood flow can quickly return.

After blood is drawn, firm pressure is applied to the site for at least five minutes or until the bleeding stops, and a wrap is placed around the draw site, which needs to be kept in place for an hour minimum. The blood sample is sent off to be analyzed in a lab.

How to prepare for an arterial blood gas test

Typically, no preparation is needed before an arterial gas test. If a patient is on supplemental oxygen therapy, the oxygen may be stopped for 20-30 minutes before the test to get an accurate reading of unassisted blood oxygen levels. However, arterial blood gas testing can be performed on supplemental oxygen therapy.

Arterial blood gas test risks

An arterial blood gas analysis tends to be more painful than a traditional blood draw done through a vein. It’s normal to experience moderate discomfort and bruising following the procedure. Patients should use a compress directly afterward to prevent bleeding.

Arterial blood gas test results

ABGs provide information about the respiratory system and the metabolic system—when one is out of balance, the other kicks in to compensate. Abnormal ABG results may signal that the person isn’t getting a high enough amount of oxygen to the body’s tissues or isn’t getting rid of enough carbon dioxide.

ABG test results are a combination of six factors—and doctors use them, along with the results of other tests, to get a full picture of the diagnosis and lung function. It’s important to keep in mind that the interpretation of an ABG is complicated. It’s the combination of these factors, rather than any one factor in particular, that may signal potential issues.

Labs may use different scales for normal values or abnormal results, so it's important to follow up with the care team to find out whether the ABG test results are in the normal range.

Below are the six factors combined in the test result.

pH level: The pH is a measure of the acids and bases in blood. Too much carbon dioxide makes it acidic, and too much oxygen increases its bases, making it more alkaline.

Partial pressure of oxygen (PaO2): A low measurement of partial pressure of oxygen, the measurement of oxygen in the blood, suggests the patient isn’t getting enough oxygen.

Partial pressure of carbon dioxide (PaCO2): A measurement of the carbon dioxide in the blood. High levels of carbon dioxide result in more acidic blood.

Oxygen saturation: This is a measurement of how much hemoglobin is present. Hemoglobin is the protein in red blood cells that carries oxygen to the body’s tissues.

Oxygen content: A measurement of how much oxygen is present per 100 mL of blood.

Bicarbonate (HCO3-): Bicarbonate is the main form of carbon dioxide in the body. This measurement indicates whether there is an imbalance in a person’s metabolic system.

Once the care team has the ABG test results, they will share them and potentially order additional tests to rule out or confirm a potential diagnosis.

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