Pulse oximetry is a non-invasive medical procedure that measures the percentage of oxygen-bound hemoglobin in the blood. This method was introduced in 1980. This is done by emitting red and infrared light from a light source (diode) to the nails, ears or tip of the nose. On the other side of the tissue, an optical sensor measures the amount of light passed through, and the device uses mathematical calculations to generate a number that indicates the percentage of hemoglobin saturation in the blood.
It's well known that most of the oxygen transfer in the blood is done by hemoglobin, so the pulse oximetry number can indicate the amount of oxygen entering the blood due to respiration and lung’s diffuse function. With adequate hemoglobin saturation in the blood (which in normal cases is more than 95 to 97%) in case of tissue’s oxygen deficiency (obtained from the TCOM test) can be concluded either:
1-    There is a disturbance in the penetration of blood oxygen into the tissue.
2-    Or, there is no adequate capillary volume in the tissue (microangiopathy)
It should be noted that the pulse oximetry device also provides the heart rate, simultaneously with the percentage of hemoglobin saturation (SpO2), which can be helpful in the process of examination and treatment. Also, the presence of nail polish, high or low blood pressure, abnormal body temperature, etc. can confuse the device.