A normal pulse oximetry reading ranges from 95 percent to 100 percent, according to Mayo Clinic. Anything below 90 percent is considered a low reading. Pulse oximeters read the percentage of oxygen in a person's blood, states Meditech. They also track a person's heart rate. Pulse oximeters were developed to help diagnose low blood oxygen, a condition called hypoxia. Sometimes the oximeter returns a false reading, either too high or too low. False readings can occur with increased patient movement, a low body temperature, anemia, in cases of shock and in cases of carbon monoxide poisoning. Nail polish can also affect the reading if the oximeter is used on a finger.