Fibrin Degradation Products Te... Health Article

Definition

When injury occurs to a blood vessel wall, thrombin (a coagulation enzyme) is formed in the first stage of a complicated series of steps called the coagulation cascade. In the second phase, fibrinogen, a coagulation protein made by the liver, is converted to fibrin, which results in the formation of a gel-like meshwork at the site. This fibrin mesh is altered by the action of factor XIIIa which cross-links fibrin polypeptides, forming a stable clot. As the site heals, the clot is broken down by the enzyme plasmin. This process, called fibrinolysis, is initiated by a protein called tissue plasminogen activator that is released from blood vessel cells adjacent to the injured site. Plasminogen activator converts plasminogen to an enzyme called plasmin. The plasmin splits polypeptides from the fibrin clot. These fragments are known as fibrin degradation products (FDP).

Fibrin degradation products are fragments (polypeptides) produced when either fibrin or fibrinogen is broken down by the enzyme plasmin. There are four principal fibrin degradation products called X, Y, D, and E that are liberated in various combinations. When a fibrin clot is broken down by plasmin, the last fragment to be degraded is one consisting of two D and one E subunits. This is split, releasing the E fragment and two D fragments that are covalently linked together. This fragment is called D-dimer, and it is produced from fibrin but not from fibrinogen degradation.

Purpose

A test for FDP may be requested by a physician when excessive bleeding occurs and thrombosis or other serious disorder in the coagulation mechanism is suspected. The FDP assay measures amounts of the fibrin and fibrinogen split products in the blood and directly indicates the level of activity of the fibrinolytic system. High levels of FDP will indicate increased fibrinolysis. Excessive fibrin degradation products are released into the plasma in three main conditions: disseminated intravascular coagulation (DIC), thromboembolytic therapy, and primary fibrinogenolysis. Fragments X, Y, E, and D are released whenever either fibrin or fibrinogen is broken down by plasmin. This degradation occurs in all three situations.

Normal blood plasma does not have significant amounts of D-dimer. It is present in the blood in detectable amounts in several conditions, most notably in disseminated intravascular coagulation (DIC), a rare disruption in normal coagulation in which rapid intramicrovascular (within the blood vessels) coagulation occurs at the same time as fibrinolysis (clot dissolution mechanism). The D-dimer test is used to diagnose DIC. It is also frequently used to help diagnose deep-vein thrombosis (clots in veins); pulmonary embolism (clots in the lungs); the thrombosis of malignancy; and sickle cell anemia (a form of anemia characterized by bleeding episodes); and to monitor the effects of thrombolytic drugs. Thrombolytic drugs that may increase D-dimer levels are barbiturates, heparin, streptokinase, and urokinase. Levels of D-dimer will be elevated in these conditions.