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→Tests Evaluating Secondary Haemostasis
==Tests Evaluating Secondary Haemostasis==
==Tests Evaluating Secondary Haemostasis==
Secondary haemostasis describes the formation of a cross linked fibrin meshwork in the blood clot and is dependent on soluble coagulation factors. Abnormalities in secondary coagulation can occur if there are insufficient coagulation factors, inactive coagulation factors or inhibition of factors. The soluble coagulation factors are traditionally divided into the intrinsic, extrinsic and common pathways, as described in the introduction.
Secondary haemostasis describes the formation of a cross linked fibrin meshwork in the blood clot and is dependent on soluble coagulation factors. Abnormalities in secondary coagulation can occur if there are insufficient coagulation factors, inactive coagulation factors or inhibition of factors. To recap, the soluble coagulation factors are traditionally divided into the intrinsic, extrinsic and common pathways.
===Activated Clotting Time===
===Activated Clotting Time===
The activated clotting time (ACT) allows rapid evaluation of secondary haemostasis. The ACT is the time taken for 2ml of fresh whole blood to clot in a tube with a contact activator (diatomaceous earth<sup>2</sup>), but an automated analyser can perform a test with a similar principle. The reaction must occur at body temperature to give a reliable indication of haemostatic ability: this can be achieved by the use of a warm water bath, or by holding the tubes under an arm. The normal ACT is 90-120 seconds and less than 75 seconds in dogs and cats respectively<sup>2</sup>.
The activated clotting time (ACT) allows rapid evaluation of secondary haemostasis. The ACT is the time taken for 2ml of fresh whole blood to clot in a tube with a contact activator (diatomaceous earth<sup>2</sup>), but an automated analyser can perform a test with a similar principle. The reaction must occur at body temperature to give a reliable indication of haemostatic ability: this can be achieved by the use of a warm water bath, or by holding the tubes under an arm. The normal ACT is 90-120 seconds and less than 75 seconds in dogs and cats respectively<sup>2</sup>.
===Activated Partial Thromboplastin Time===
===Activated Partial Thromboplastin Time===
The APTT is measures the time necessary to generate fibrin from activation of the intrinsic pathway<sup>3</sup>. It therefore assesses functionality of the components of the intrinsic and common pathways of coagulation. The test is performed on citrated plasma, and so blood should be collected into a sodium citrate tube if the APTT test is to be undertaken. Once a sample is obtained, factor XII is activated by an external agent that will not also activate factor VII, such as kaolin<sup>1, 3</sup>. Since the intrinsic arm of the cascade requires platelet factors to function, the test also provides a phospholipid emuslion in place of these factors. Calcium is added, the preparation is incubated, and the time for clumping of kaolin is measured. Classically, partial thromboplastin time was measured after activation by contact with a glass tube, but use of an external activating agent in the newer, "activated" partial thromboplastin time method makes results more reliable<sup>3</sup>.
The APTT is measures the time necessary to generate fibrin from activation of the intrinsic pathway<sup>3</sup>. It therefore assesses functionality of the components of the intrinsic and common pathways of coagulation. The test is performed on citrated plasma, and so blood should be collected into a sodium citrate tube if the APTT test is to be undertaken. Once a sample is obtained, factor XII is activated by an external agent that will not also activate factor VII, such as kaolin<sup>1, 3</sup>. Since the intrinsic arm of the cascade requires platelet factors to function, the test also provides a phospholipid emuslion in place of these factors. Calcium is added, the preparation is incubated, and the time for clumping of kaolin is measured. Classically, partial thromboplastin time was measured after activation by contact with a glass tube, but use of an external activating agent in the newer, "activated" partial thromboplastin time method makes results more reliable<sup>3</sup>.
===Prothrombin Time===
===Prothrombin Time===
Prothrombin time (PT) gives an assessment of the extrinsic and common pathways by measuring the time necessary to generate fibrin after activation of factor VII<sup>3</sup>. It is performed by an automated analyser<sup>2</sup> using citrated plasma<sup>1, 3</sup>. Blood should therefore be collected into a sodium citrate tube if prothrombin time is to be performed. In basic terms, the test procedure involves adding thromoplastin to the patient's plasma, warming, adding calcium and recording the time taken to clot<sup>1</sup>.
Prothrombin time (PT) gives an assessment of the extrinsic and common pathways by measuring the time necessary to generate fibrin after activation of factor VII<sup>3</sup>. It is performed by an automated analyser<sup>2</sup> using citrated plasma<sup>1, 3</sup>. Blood should therefore be collected into a sodium citrate tube if prothrombin time is to be performed. In basic terms, the test procedure involves adding thromoplastin to the patient's plasma, warming, adding calcium and recording the time taken to clot<sup>1</sup>.
===Tests for Individual Clotting Factors===
===Tests for Individual Clotting Factors===
Some specialised laboratories offer tests for specific clotting factors. Blood is required to be collected into a sodium citrate tube.
Some specialised laboratories offer tests for specific clotting factors. Blood is required to be collected into a sodium citrate tube.
===Proteins Induced by Vitamin K Antagonism Test===
===Proteins Induced by Vitamin K Antagonism Test===
Factors II, VII, IX and X are produced in the liver as non-functional precursors which become activated by carboxylation of their glutamic acid residues in the presence of vitamin K. The inactive precursors are absent from the circulation of normal animals, as they are stored in the microsomal system of the liver. However, the absence of vitamin K results in an increase in these precursors, which spill into the circulation and become known as Proteins Induced by Vitamin K Antagonism (PIVKA). Concurrently, the levels of active factors II, VII, IX and X are depleted.
Factors II, VII, IX and X are produced in the liver as non-functional precursors which become activated by carboxylation of their glutamic acid residues in the presence of vitamin K. The inactive precursors are absent from the circulation of normal animals, as they are stored in the microsomal system of the liver. However, the absence of vitamin K results in an increase in these precursors, which spill into the circulation and become known as Proteins Induced by Vitamin K Antagonism (PIVKA). Concurrently, the levels of active factors II, VII, IX and X are depleted.