An arterial blood gas (ABG) is a blood test that is performed using blood from an artery. It involves puncturing an artery with a thin needle and syringe and drawing a small volume of blood. The most common puncture site is the radial artery at the wrist, but sometimes the femoral artery in the groin or other sites are used. The blood can also be drawn from an arterial catheter. Pulse oximetry plus transcutaneous carbon dioxide measurement is an alternative method of obtaining similar information as well. An ABG is a test that measures the arterial oxygen tension (PaO2), carbon dioxide tension (PaCO2), and acidity (pH). In addition, arterial oxyhemoglobin saturation (SaO2) can be determined. Such information is vital when caring for patients with critical illness or respiratory disease. As a result, the ABG is one of the most common tests performed on patients in intensive care units (ICUs).

    Arterial blood for blood gas analysis is usually drawn by a respiratory therapist and sometimes a phlebotomist, nurse or doctor. Blood is most commonly drawn from the radial artery because it is easily accessible, can be compressed to control bleeding, and has less risk for occlusion, the selection of which radial artery to draw from is based on the outcome of an Allen's test. The femoral artery (or less often, the brachial artery) is also used, especially during emergency situations or with children. Blood can also be taken from an arterial catheter already placed in one of these arteries.

    The syringe is pre-packaged and contains a small amount of heparin, to prevent coagulation or needs to be heparinised, by drawing up a small amount of heparin and squirting it out again. Once the sample is obtained, care is taken to eliminate visible gas bubbles, as these bubbles can dissolve into the sample and cause inaccurate results. The sealed syringe is taken to a blood gas analyzer. If the sample cannot be analyzed within 10-15 minutes, it must be placed on ice for valid results. Even when placed on ice, samples should still be analyzed within 1 hour.

    Standard blood tests can also be performed on arterial blood, such as measuring glucose, lactate, hemoglobins, dys-haemoglobins, bilirubin and electrolytes.