Venipuncture is the collection of blood from a vein, usually for laboratory testing.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.

Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

How you prepare depends on the specific blood test you are having done. Many tests do not require any special preparation. Other times, you may be told to avoid food or drinks or limit certain medications before the test.

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.

Blood is made up of two parts:

• Fluid (plasma or serum)
• Cells

Plasma contains various substances. Serum is the fluid part that remains after the blood is allowed to clot in a test tube. Specifically, serum is the fluid part of blood after a substance called fibrinogen has been removed.

Cells in the blood include red blood cells, white blood cells, and platelets.

Blood helps move oxygen, nutrients, waste products, and other materials through the body. It helps control body temperature, fluid balance, and the body's acid-base balance.

Tests on blood or parts of blood may give your doctor important clues about your health.

Normal results vary with the specific test.

Abnormal results vary with the specific test.

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.

Other risks associated with having blood drawn are slight but may include:

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