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Coagulation is a fairly complex process by which liquid blood becomes solid clots. It is an important part of hemostasis where a damaged blood vessel is ultimately covered by a fibrin clot to stop blood loss. Disorders in coagulation can lead to hemorrhage, thrombosis, or embolism.

Ordinarily coagulation is initiated within seconds after an injury occurs when platelets form a plug at the site of injury. This is called primary hemostasis. Following this, various plasma components, called clotting factors respond (in a complex cascade) to form fibrin strands which strengthen the platelet plug.

Table of contents
1 Primary hemostasis
2 The coagulation cascade
3 Disorders of Hemostasis
4 Coagulation factors
5 Laboratory tests of coagulation

Primary hemostasis

Primary hemostasis is initiated when platelets adhere, using a specific platelet collagen receptor glycoprotein Ia/IIa, to collagen fibers in vascular epithelium. This adhesion is stabilized by von Willebrand's factor which forms links between the platelet glycoprotein Ib/IX and collagen fibrils.

The platelets are then activated to secrete the contents of their granules in to the plasma, which causes a change in their shape. Fibrinogen, which links adjacent platelets by forming links via the glycoprotein IIb/IIIa.

The different pathways of the coagulation cascade.

The coagulation cascade

The coagulation cascade of secondary hemostasis is traditionally divided into three parts: the final common pathway, the intrinsic pathway (abnormal in ) and the extrinsic pathway. The pathways are a series of reactions, in which a stable form of a protein is activated to become an enzyme which then catalyzes the next reaction in the cascade. Coagulation factors are generally indicated by
Roman numerals, with a lowercase a appended to indicate an active form, ultimately resulting in cross-linked fibrin.

The intrinsic pathway is initiated by activation of contact factors of plasma, and can be measured by the prothrombin time (PT) test, sometime reported as an INR value.

The extrinsic pathway is initiated by exposure of blood to "tissue factor" (a specific cellular lipoprotein), and can be measured by the activated partial thromboplastin time (PTT) test.

The common pathway is reached by completion of either or both of the above pathways, and results in the elaboration of thrombin.

If a coagulation factor is part of the intrinsic or extrinsic pathway, a deficiency of that factor will affect only one of the tests: thus hemophilia A, a deficiency of factor VIII, which is part of the intrinsic pathway, results in an abnormally prolonged PT test but a normal PTT test.

Disorders of Hemostasis

Coagulation factors

Laboratory tests of coagulation

Common: APTT, INR (PT), TCT
Other: factor assays, mixing test, Lupus anticoagulant, genetic tests, dilute Russell viper venom test (dRVVT),
bleeding time