Arterial blood gas (ABG) is a test that measures various capacities of gases to enter or exit the bloodstream, the saturation of oxygen in the blood, and materials involved in maintaining the pH level of blood. It is widely considered one of the most useful diagnostic tools for a host of circulatory conditions, and is a standard procedure for many hospital stays.

Hematocrit cannot be measured by an arterial blood gas test.

Blood is around 55% plasma, 45% erythrocytes (red blood cells), and less than 1% of leukocytes and platelets.