Blood performs many vital functions in your body. It transports oxygen, fluids and nutrients, and regulates body temperature. Tests on blood or its components may offer valuable information in diagnosing and managing disease.

The term blood refers to whole blood, which contains water, proteins, nutrients and blood cells. Blood consists of two elements that your doctor may want to test:

• The cell portion, which includes red blood cells, white blood cells and platelets
• The fluid portion, which is called plasma or serum

Plasma is mostly water but also contains minerals and proteins. It's obtained by separating the liquid portion of unclotted blood from the cells. Serum is the fluid that remains after the blood has clotted. It contains no cells and, unlike plasma, no dissolved proteins.

Whether plasma or serum is used for a blood test depends on the type of blood test ordered and what the doctor is trying to find out.