A level between 95 and 100 percent usually constitutes healthy blood oxygen saturation, according to WebMD. An oxygen saturation level of less than 90 percent is abnormally low. A device called a pulse oximeter measures blood oxygen saturation and estimates a patient's blood oxygen level.
There is a difference between blood oxygen saturation and blood oxygen level, notes WebMD. Although physicians often consider a blood oxygen saturation reading from a pulse oximeter to be a reliable estimate of the patient's blood oxygen level, they can directly measure blood oxygen with a blood sample from an artery. This method of measuring oxygen levels provides information on the quantity of oxygen flowing through the patient's arteries.