An arterial blood gas (ABG) test is a blood gas test of blood from an artery; it is thus a blood test that measures the amounts of certain gases (such as oxygen and carbon dioxide) dissolved in arterial blood. An ABG test 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 thewrist, but sometimes the femoral artery in the groin or other sites are used. The blood can also be drawn from an arterial catheter. An ABG test measures the blood gas tension values of arterial oxygen tension (PaO2), arterial carbon dioxide tension (PaCO2), and acidity (pH). In addition, arterial oxygen saturation (SaO2) can be determined. Such information is vital when caring for patients with critical illness or respiratory disease.