The ABG or Arterial Blood Gas Analysis test measures oxygen levels, carbon dioxide levels, and acidity, or pH, in the blood. The blood sample is usually taken from an artery in the wrist. ABG testing is done to detect / monitor lung and kidney problems to check the body’s acid-base balance. It is mostly done for patients who are in ICU and are on ventilator. In these patients lungs and kidneys get severely affected leading to problem with the pH levels of the blood.