Prothrombin Time (PT)

Special Considerations

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.