SpO2, also known as oxygen saturation, is a measure of the amount of oxygen-carrying hemoglobin in the blood relative to the amount of hemoglobin not carrying oxygen. It is a percentage of the maximum amount of oxygen that the blood could carry. A pulse oximeter is a non-invasive device that measures SpO2 levels through a small clip that is usually placed on a finger or toe. The device measures light wavelengths to determine the ratio of the current levels of oxygenated hemoglobin to deoxygenated hemoglobin. A normal SpO2 level for a healthy individual should be between 96% to 99%. However, high altitudes and other factors may affect what is considered normal for a given individual. Pulse oximetry is often regarded as the fifth vital sign and is used to monitor patients before, during, and after surgery, including during anesthesia, and to monitor patients on certain medications that may reduce respiration and lung function. It is also used to monitor patients with lung and heart disorders who are at risk of low levels of blood oxygen.