Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the band to fill with blood.

A 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.

For infants or young children:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

• infant test or procedure preparation (birth to 1 year)
• toddler test or procedure preparation (1 to 3 years)
• preschooler test or procedure preparation (3 to 6 years)
• schoolage test or procedure preparation (6 to 12 years)
• adolescent test or procedure preparation (12 to 18 years)

• congenital deficiency of factor II
• fatmalabsorption
• liver disease (such as cirrhosis)
• Vitamin K deficiency
• warfarin administration

• DIC (disseminated intravascular coagulation)

• 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

Coagulation (blood clotting) results from a sequence of reactions involving several proteins known as coagulation factors. Some of these factors have other names. For example, Factor I is also called fibrinogen, Factor II is prothrombin, and Factor XII is Hageman factor. The liver produces these proteins and secretes them into the blood. In addition, vitamin K is important to blood clotting because your body converts it into prothrombin.

Some people take warfarin to keep their blood from clotting. Warfarin inhibits prothrombin, thus interrupting the clotting cascade. Because of the link between vitamin K and prothrombin, people who take warfarin need to have consistent levels of vitamin K in their diet, as instructed by their doctor.

Coagulation begins when some of the coagulation factors contact damaged tissue. Each factor reaction triggers the next reaction, in a cascade. The final product of the coagulation cascade is the blood clot.

Substances that inhibit or "turn off" the action of the coagulation factors are activated along with the coagulation factors. These substances function more slowly, over a longer period of time, than the coagulation factors. This allows a clot to form, stopping the bleeding, then, after enough time has passed for the tissue to heal, the clot is dissolved and blood flow restored. The most important coagulation inhibitor is antithrombin III.

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.