Pulse oximetry is a non-invasive method which allows health care providers to monitor the oxygenation of a patient's blood.

A sensor is placed on a relatively thin part of the patient's anatomy, usually a fingertip or earlobe, and red and infrared light is passed from one side to the other. Based upon the ratio of absorption of the red and infrared light caused by the difference in color between oxygen-bound (red) and unbound (blue) hemoglobin in the capillary bed, an approximation of oxygenation can be made.

This is especially useful in an intensive care setting, for asssessment of emergency patients, determining the effectiveness of or need for supplemental oxygen, and monitoring during anesthesia.

It should be noted that this is a measure solely of oxygenation, not of ventilation, and is not a substitute for blood gases checked in a laboratory as it gives no indication of carbon dioxide levels, blood pH, or sodium bicarbonate levels.

Falsely low readings may be caused by hypoperfusion of the extremity being used for monitoring, often due to the part being cold or from vasoconstriction secondary to the use of vasopressor agents. Falsely high readings will occur when hemoglobin is bound to something other than oxygen, such as in cases of carbon monoxide poisoning.