PT

Alternative names:

pro-time; prothrombin time

What the risks are:

• excessive bleeding
• fainting or feeling lightheaded
• hematoma (blood accumulating under the skin)
• infection (a slight risk any time the skin is broken)
• multiple punctures to locate veins

Note: This test is often performed on people who may have bleeding problems; if so, the risk of bleeding and hematoma are slightly greater than for people without bleeding problems.

Special considerations:

Coagulation results from a sequence (cascade) of reactions involving coagulation factors, for example, factor I, factor II. Some of these factors have other names, for example, Factor I (fibrinogen), Factor II (prothrombin), and Factor XII (Hageman factor). These proteins are produced in the liver and secreted into the blood. Some of the Factors (that is, II, VII, IX, and X) require vitamin K for their synthesis. Warfarin (Coumadin) is a commonly-used anticoagulant drug. It acts by inhibiting the vitamin K-requiring enzyme in the liver, which prevents formation of some of the clotting factors, thereby inhibiting coagulation.

When clotting factors contact damaged tissue, the coagulation sequence is initiated. Each clotting factor reaction triggers the next reaction. The final product of the cascade is the fibrin clot (blood clot).

Factor X can be activated by 2 separate sequences of chemical reactions. The factors involved in the 2 sequences are referred to as the intrinsic system and the extrinsic system. The intrinsic system involves activation of Hageman factors by tissue not normally in contact with blood, followed by sequential activation of factors XI, IX, and X, in the presence of factor VIII. The extrinsic system simply involves activation of factor VII by thromboplastin (also called tissue factor), which is a protein released from the membranes of damaged tissues.

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

Interfering factors
Anticoagulant therapy (heparin, warfarin)

Drugs that can increase PT times include allopurinol, aminosalicylic acid, aspirin, beta lactam antibiotics, cephalothins, chloramphenicol, chlorpromazine, cholestyramine, cimetidine, clofibrate, colestipol, Coumadin, methyldopa, neomycin, propylthiouracil, quinidine, quinine, salicylates and sulfonamides.

Drugs that can decrease PT times include anabolic steroids, digitalis, diphenhydramine, estrogens, griseofulvin, and oral contraceptives.

Increased red blood cell mass. This results in excess anticoagulant in the serum (from the anticoagulant in the vial used to collect the sample), 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.