Both pulse oximetry and arterial blood gas testing measure the amount of oxygen in the blood, which helps determine how well the lungs are functioning. Pulse oximetry is not invasive. It uses a sensor attached to the person's finger. It can also provide continuous measurement and monitoring of the amount of oxygen in the blood. Arterial blood gas tests are invasive, requiring a blood sample from an artery, and provide information at a specific moment in time.
(See also Medical History and Physical Examination for Lung Disorders.)
Pulse oximetry
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.
Doctors may do pulse oximetry as or after the person walks around or climbs a flight of stairs to see whether exertion causes oxygen levels in the blood to decrease.
Arterial blood gas measurement
An arterial blood gas measurement can give a more exact measurement of the blood oxygen level than pulse oximetry, and provides other information as well. Arterial blood gas tests measure the levels of oxygen and carbon dioxide in the arterial blood and determine the acidity (pH) of the blood. 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, how well air is moving in and out of the lungs, and how the breathing may be affecting the body overall.
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).
There are other ways of measuring carbon dioxide in exhaled breath that do not require blood samples, but these methods are sometimes less accurate and not always easily available.
