Blood is drawn from a vein, usually on 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 an infant or young child:
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)

• bile duct obstruction
• cirrhosis
• disseminated intravascular coagulation
• hepatitis
• malabsorption (inadequate absorption of nutrients from the intestinal tract)
• Vitamin K deficiency
• Coumadin (warfarin) therapy
• Factor VII deficiency
• Factor X deficiency
• Factor II (prothrombin) deficiency
• Factor V deficiency
• Factor I (fibrinogen) deficiency

This test may be performed under many conditions in the evaluation of a wide variety of disorders.

• 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.

Factor X can be activated by two different chemical-reaction sequences. The factors involved in the two sequences are referred to as the intrinsic system and the extrinsic system. In the extrinsic system, a substance called thromboplastin or tissue factor (a protein released by damaged tissues) activates factor VII.

The PT test is used to evaluate the adequacy of the extrinsic system. It measures the clotting ability of factors I (fibrinogen), II (prothrombin), V, VII, and X. When any of these factors is deficient, the PT is prolonged.

Interfering factors:

• Anticoagulant therapy (heparin, warfarin).
• Increased red blood cell mass. This results in excess anticoagulant in the serum, resulting in artificially prolonged PT.
• 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.