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Factor XII Deficiency

Definition

Factor XII is a deficiency in one of the proteins (Factor XII [12]) involved in clotting blood.

Deficiency of factor XII is considered a rare clotting disorder, and it involves no clinical bleeding.

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Blood Coagulation

Blood coagulation is the process whereby the components of the liquid blood are transformed into a semisolid material called a blood clot.

The purpose of coagulation is obvious - to plug up ruptured vessels so as to stop bleeding and prevent loss of vital body fluid.

The blood clot is made up mainly of blood cells entrapped in a meshwork of fibrin. Fibrin is formed from proteins in the plasma as the result of a complex series of reactions. At least 12 clotting factors are involved in the reaction that forms fibrin.

The blood-coagulation mechanism presumably consists of a series of chemical reactions that take place in a definite and rapid sequence. When tissue is injured, it starts the first of these changes that initiate blood clotting.

The trigger that starts it is the appearance of a "rough" spot in the lining of a blood vessel. Within a matter of 1 or 2 seconds, clumps of platelets adhere to any portion of a blood vessel that loses its normal, perfectly smooth quality and releases a variety of substances.

After the platelets release its substances, a series of chemical reactions take place in a rapid-fire succession. Some of these substances that accelerate coagulation are Factors VII, VIII, IX, X, XI, and XII. Factor XII is also referred to as the Hageman factor (after a patient named John Hageman).

The sequence of events that leads to the generation of thrombin and the formation of a fibrin clot is a biochemical cascade and consists of two distinct pathways, extrinsic (coming from outside) and intrinsic (coming from within, not dependent on externals).

In the intrinsic pathway, Factor XII is activated during the contact phase of blood clotting, followed by the sequential activation of factors IX, X, and XI, and prothrombin.

The extrinsic pathway is also activated. This is the initial blood clotting mechanism and is the only known time when Factor XII affects coagulation.

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Factor XII

Factor XII deficiency was first described in 1968 by Ratnoff. The factor deficiency has been shown to be an inherited autosomal recessive disorder.

The presence of Hageman factor (XII) abnormality trait may be suspected in a person with prolonged activated partial thromboplastin time (APTT), normal prothrombin time (PT), normal bleeding time, and no history of bleeding. Despite the dramatic prolongation of the APTT, deficient individuals have no bleeding problems and normal hemostasis.

Because there is no history of bleeding, the diagnosis of Factor XII deficiency is often made incidentally when routine laboratory evaluation is done.

It is important to recognize and diagnose these disorders since the patients should neither be inappropriately treated with plasma nor denied indicated surgery on the basis of these laboratory abnormalities.

The absence of bleeding in people with Factor XII deficiency remains unexplained. At least two major hypotheses have been proposed: (1) Factor XII may be absent or decreased in serum but may be present in other tissues; and (2) alternate coagulation pathways are present that may bypass Factor XII mediated coagulation.

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Questions to Ask Your Doctor

Will this deficiency lead to any other complications?

Will you be prescribing any medication? What are the side-effects?

It is hereditary? If so, what signs and/or symptoms will occur in family members?

Does having blood drawn for lab work present any problems?

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