While the finger pulse oximeter is a popular means of monitoring for SPO2, it is actually not the best location. True blood oxygen can only be determined by a blood gas measurement, involving a blood draw and laboratory equipment. Because this isn't feasible in the field, the oximeter offers a more convenient method using absorption of red and infrared light to estimate the actually blood oxygen level. The oximeter then is just that - an estimation of true blood oxygen. There is actually a difference in the quality of the estimation depending upon the location where the pulse oximeter measures the light absorption. As mentioned, one popular place is the finger, but there is also the forehead and the ear area. This particular study shared by Aithre customer, glider pilot, and economics professor at Stanford Law School, reviews the different oximeter locations and provides hard data on the correlation between the estimation and the true blood oxygen levels. Conclusion is that the finger is the worst place to estimate true blood oxygen and can be off by many percentage points and fluctuate wildly. The ear location and the forehead are the preferred locations for oximeter placement and offer estimations the are very tightly correlated to true blood oxygen.