* Pulse oximetry is a non-invasive method that uses a small sensor placed on the finger, earlobe, or forehead. The sensor emits light that passes through the skin and blood vessels, and the amount of light absorbed by the blood is measured. This measurement is used to calculate the oxygen saturation of the blood, which is the percentage of hemoglobin molecules that are carrying oxygen.

* Arterial blood gas (ABG) analysis is a more invasive method that involves drawing a sample of blood from an artery. The blood sample is then analyzed in a laboratory to measure the oxygen partial pressure (PaO2), which is the amount of oxygen dissolved in the blood.

* Transcutaneous oxygen monitoring (TcO2) is a method that uses a small electrode placed on the skin to measure the partial pressure of oxygen in the skin tissue. This method is commonly used in newborns and critically ill patients.

The goal of oxygen level measurement is to ensure that the body is receiving enough oxygen. Low oxygen levels can lead to a variety of health problems, including shortness of breath, confusion, and even death.