Most of the time, blood is drawn from a vein located on the inside of the elbow or the back of the hand.

• The site is cleaned with germ-killing medicine (antiseptic).
• An elastic band is put around the upper arm to apply pressure to the area. This makes the vein below it swell with blood.
• A needle is inserted into the vein.
• The blood collects into an airtight vial or tube attached to the needle.
• The elastic band is removed from your arm.
• The needle is taken out and the spot is covered with a bandage to stop 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 onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

The steps you need to take before the test will depend on the kind of blood test you are having. Many tests do not require special preparation.

In some cases, your health care provider will tell you if you need to stop taking any medicines before you have this test or if you need to be fasting. Do not stop or change your medicines without talking to your provider first.

You may feel slight pain or a sting when the needle is inserted. You may also feel some throbbing at the site after the blood is drawn.

Blood is made up of two parts:

• Fluid (plasma or serum)
• Cells

Plasma is the fluid part of the blood in the bloodstream that contains substances such as glucose, electrolytes, proteins, and water. Serum is the fluid part that remains after the blood is allowed to clot in a test tube.

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 provider important clues about your health.

Normal results vary with the specific test.

Abnormal results vary with the specific test.

Blood-draw; Phlebotomy

Dean AJ, Lee DC. Bedside laboratory and microbiologic procedures. In: Roberts JR, Custalow CB, Thomsen TW, eds. Roberts and Hedges' Clinical Procedures in Emergency Medicine and Acute Care. 7th ed. Philadelphia, PA: Elsevier; 2019:chap 67.

Feirstein J. Venipuncture. In: Dehn R, Asprey D, eds. Essential Clinical Procedures. 4th ed. Philadelphia, PA: Elsevier; 2021:chap 31.

Patel K, Jones PM. Specimen collection and processing. In: Rifai N, Chiu RWK, Young I, Burnham Carey-Ann D, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. St Louis, MO: Elsevier; 2023:chap 4.

Updated by: Jacob Berman, MD, MPH, Clinical Assistant Professor of Medicine, Division of General Internal Medicine, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.