An arterial blood gas (ABG) study is a blood test that measures the levels of many different gases in oxygen-rich blood. Some of these levels are measured directly while others are calculated from the measurements of other gases. By measuring the gases in arterial blood, a physician can determine its contents before the blood nourishes the body. The analysis of arterial gases provides the physician with information about:

• How well the body’s lungs are working during or after a procedure (e.g., open-heart surgery, which requires the use of a heart-lung machine).
  
  
• Whether a ventilator (a device that breathes for a patient) is set to deliver the proper amount of oxygen needed by a patient.
  
  
• How well the body’s metabolic system is working.
  
  
• Whether the body’s acid/base levels are in balance.
  
  
• The effectiveness of respiratory therapy and other treatments for patients with chronic heart, lung or metabolic disorders.
  
  
• Whether a premature infant is getting too much oxygen, which can cause blindness or lung disorders.
  
  
• How well the kidneys are functioning. 

For the test, blood will be drawn from an artery, usually the radial artery on the side of the wrist. Other possible sites are the brachial artery (in the arm) and femoral artery (in the groin).

Before the test, a healthcare professional will use a piece of cotton to clean the area and will answer any questions that the patient may have. The healthcare professional will then insert a small needle into the artery to draw blood into an attached syringe. Because the blood is being drawn from a blood vessel that has more nerves and is thicker than a vein, the patient may experience more pain than from a standard blood test. It is not uncommon to feel a throbbing sensation after the test is over.

Once the needle is removed, the healthcare provider will apply a small patch of cotton over the area and will typically ask the patient to apply gentle pressure to the area. After about five to 10 minutes, any minor bleeding should have stopped.

In the meantime, the blood sample is swiftly taken to a laboratory to be tested. One type of machine measures partial pressure of oxygen (PaO2), partial pressure of carbon dioxide (PaCO2) and pH levels. Another machine measures hemoglobin levels. From these measurements, other important values can be calculated.

The typical ABG test will list the following measurements:

• PaO2 (partial pressures of oxygen). This measure is an indication of how well the lungs are infusing blood with fresh oxygen. At sea level, normal levels of this gas in arterial blood are 75 to 100 millimeters of mercury (mmHg).
  
  
• PaCO2 (partial pressures of carbon dioxide). This measure is an indication of how well the lungs are removing carbon dioxide from the blood. At sea level, normal levels of this gas in arterial blood are 35 to 45 mmHg.
  
  
• The pH level. This is a measure of acid (acidity) and base (alkalinity) on a scale of 0 to 14, with “7” being a neutral balance between acid and base. Normal pH levels of arterial blood are 7.35 to 7.42.
  
  
• O2CT (oxygen content). Normal oxygen content in arterial blood is 15 to 23 percent.
  
  
• SaO2 (oxygen saturation). Normal oxygen saturation in arterial blood is 94 to 100 percent.
  
  
• HCO3 (bicarbonate). Normal levels of this gas in arterial blood are 22 to 26 milliequivalents per liter (mEq/liter).