Both arterial blood gas testing and pulse oximetry measure the amount of oxygen in the blood, which helps determine how well the lungs are functioning. Arterial blood gas tests are invasive, requiring a blood sample, and provide information at a specific moment in time. Pulse oximetry is not invasive. It uses a sensor attached to the person's finger. It can also provide continuous measurements of the amount of oxygen in the blood.
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. There are newer ways of measuring carbon dioxide in exhaled breath that do not require blood samples, but these methods are less accurate and not always available.
The amount of oxygen in 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. An arterial blood gas measurement can also give a more exact measurement than pulse oximetry.
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.