Alternative Names: Blood-drawing; Phlebotomy

How the test is performed:

Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the elastic band to distend (fill with blood). A sterile needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

How to prepare for the test:

Preparation may vary depending on the specific test, and many tests do not require any special preparation. The health care provider may limit certain medications shortly before the test or require an overnight fast to assure an accurate sample.

How the test will feel:

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

Why the test is performed:

Blood is used by the body for transportation of oxygen, food, waste products, and other materials within the body. It is also used for regulation of body temperature, regulation of fluids, and acid-base equilibrium. Because blood is used for multiple functions within the body, tests on the blood or its components may give valuable clues in the diagnosis of a multitude of medical conditions.

Blood is made up of a fluid portion (plasma) and a cellular portion. Plasma contains various substances dissolved in the fluid. Serum is what remains when fibrinogen has been removed from the plasma (this is the fluid that remains after blood is allowed to clot in a test tube). The cellular portion consists primarily of red blood cells, but also includes white blood cells and platelets.

Normal Values:

Normal results vary with the specific test.

What abnormal results mean:

Abnormal results vary with the specific test.

What the risks are:

Risks associated with venipuncture are slight:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Special considerations:

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.