Arterial blood gas tests measure the levels of oxygen and carbon dioxide in the arterial blood and determine the acidity (pH) of the blood. Taking a blood sample from an artery using a needle may cause a few minutes of discomfort. Usually the sample is taken from an artery in the wrist (radial artery). Oxygen, carbon dioxide, and acidity levels are important indicators of lung function because they reflect how well the lungs are getting oxygen into the blood and getting carbon dioxide out of it.
Oxygenation of the blood can be monitored without taking a blood sample by using a sensor placed on a finger or an earlobe—a procedure called pulse oximetry. However, when a doctor also needs a carbon dioxide or blood acidity measurement (for example, in certain people who are seriously ill), an arterial or venous blood gas measurement is usually needed. There are newer ways of measuring carbon dioxide that do not require blood samples, but these methods are less accurate and not always available.
Doctors may do pulse oximetry as or after the person walks around or climbs a flight of stairs to see if exertion causes oxygen levels in the blood to decrease.