Why antithrombin 3 is decreased and thrombin - antithrombin complex is increased in DIC?

Disseminated Intravascular Coagulation (DIC) is a complex and serious condition characterized by widespread activation of blood clotting (coagulation) throughout the body. In DIC, the balance between procoagulant and anticoagulant factors is disrupted, leading to the formation of excessive blood clots (thrombi) in small blood vessels, as well as the consumption of clotting factors and platelets.

Antithrombin III (AT-III) is a natural anticoagulant that inhibits several clotting factors, including thrombin. In DIC, the levels of antithrombin III are often decreased. There are a few reasons for this:

  1. Consumption: The widespread activation of coagulation in DIC leads to the consumption of clotting factors, including antithrombin III. As the body tries to control the excessive clotting, antithrombin III is used up in the process.
  2. Inhibition by fibrin clots: Fibrin clots, which are formed during the coagulation process, can bind and sequester antithrombin III. This means that even if there is some antithrombin III present, it may be less effective in inhibiting clotting factors due to its interaction with fibrin clots.

The Thrombin-Antithrombin (TAT) complex is a marker of thrombin generation. Thrombin is a key enzyme in the blood clotting cascade, and in DIC, there is an increased production of thrombin due to the widespread activation of coagulation. The formation of the Thrombin-Antithrombin complex occurs when thrombin binds to antithrombin III. As antithrombin III levels decrease in DIC, there is a relative excess of thrombin, leading to an increased formation of the Thrombin-Antithrombin complex.

In summary, the decrease in antithrombin III levels in DIC is a result of consumption and binding to fibrin clots, while the increase in the Thrombin-Antithrombin complex reflects the elevated thrombin generation in the setting of DIC. These imbalances contribute to the pathological clotting seen in DIC.