Blood Oximetry is an easy non-invasive way to monitor Oxygen Saturation in blood (blood bound to hemoglobin, percentage of red blood cells carrying oxygen).
SPO2 = Saturation of Peripheral Oxygen : displayed on Oximeter
SaO2 = Saturation of Arterial Oxygen gas in blood : measured via pulling blood and analyzing it.
SO2 = Saturation of Oxygen (% of hemoglobin sites bound to oxygen).
SPO2 is not necessarily same as SaO2 but they generally correlate so well (exceptions: CO-poisoning where CO attaches to hemoglobin rather than oxygen, people with lung disorders, anemia, vasoconstriction, cold extremities, heavy nail polish on fingers where oximeter sensor is placed, cigarette smoking etc.) that it can be clinically used to monitor body oxygen levels.
Red blood cells contain a protein called Hemoglobin. When a hemoglobin carries its maximum 4 oxygen molecules, it is “saturated”. Nearly all blood oxygen is hemo-bound when passing through the lungs. A healthy person (under normal conditions) has a SPO2 of 95-100%.
Oximeter is a small sensor worn on a finger (or sometimes on foot or earlobe), it sends 2 light beams (infrared and red) via 2 different LEDs through the (thin part of the) body. Blood saturated with oxygen aborbs less red than unsaturated blood. The amount of unabsorbed lights is measured & by way of calculation of ratio of oxygenated to deoxygenated hemoglobin using mathematical tables/laws, the SPO2 is displayed. To insure a good reading is better not to wear nail polish, be relaxed with warm hands below heart level.