How reliable are pulseoximeters measuring oxygen levels?

Fairly reliable: The oxygen level from a pulse oximeter is reasonably accurate. Most oximeters give a reading 2% over or 2% under what your saturation would be if obtained by an arterial blood gas. For example, if your oxygen saturation reads 92% on the pulse oximeter, it may be actually anywhere from 90 to 94%. If your pulse oximeter has an index reading, it can tell you the relability of any one reading.