Changes

The contact activator used in the ACT test triggers the intrinsic pathway, and so ACT allows assessment of the intrinsic and common pathways. ACT will therefore be prolonged when factors I, II, V, VIII, IX, X, XI or XII are deficient or abnormal, such as in DIC, liver disease, vitamin K antagonist toxicosis or haemophilia A or B². Thrombocytopenia may also increase ACT.

The contact activator used in the ACT test triggers the intrinsic pathway, and so ACT allows assessment of the intrinsic and common pathways. ACT will therefore be prolonged when factors I, II, V, VIII, IX, X, XI or XII are deficient or abnormal, such as in DIC, liver disease, vitamin K antagonist toxicosis or haemophilia A or B². Thrombocytopenia may also increase ACT.

===Activated Partical Thromboplastin Time===  

===Activated Partial Thromboplastin Time===  

The APTT is measures the time necessary to generate fibrin from activation of the intrinsic pathway³. 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^{,1 3}. 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 reliable³.

The APTT is measures the time necessary to generate fibrin from activation of the intrinsic pathway³. 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^{,1 3}. 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 reliable³.

 

APPT evaluates the same pathways as ACT, and so will be prolonged by abnormalities or deficiencies in factors XII, XI, IX, VIII, X, V, II or I. However, is not affected by thrombocytopenia and is also considered to be a more sensitive test than ACT: APTT becomes prolonged when 70% of a factor is depleted, compared to 90% depletion of ACT.

Like the ACT it will be prolonged by abnormalities or

deficiencies in factors XII, XI, IX, VIII, X, V, II or I. It is not affected by thrombocytopenia. The APTT is considered

a more sensitive test than the ACT and will become prolonged when approximately 70% of a factor is depleted in

comparison the ACT will only be prolonged when approximately 90% of a factor is depleted.

This test is abnormal in the presence of reduced quantities of factors XII, IX, XI, VIII, X, V, prothrombin, and fibrinogen (all integral parts of the "intrinsic" and "common" pathway. It is usually prolonged if a patient has less than approximately 30% normal activity. It can also be abnormal in the presence of a circulating inhibitor to any of the intrinsic pathway factors. The differentiation of inhibitors from factor depletion is important and can best be accomplished by a mixing study in which patient and normal plasma are combined in a 1:1 ratio and the test is repeated on the mixed sample. If the abnormal value is corrected completely, the problem is factor deficiency. If the result does not change or the abnormality is corrected only partially, an inhibitor is present. This difference stems from the above mentioned fact that the aPTT will be normal in the presence of 50% normal activity.

This test is abnormal in the presence of reduced quantities of factors XII, IX, XI, VIII, X, V, prothrombin, and fibrinogen (all integral parts of the "intrinsic" and "common" pathway. It is usually prolonged if a patient has less than approximately 30% normal activity. It can also be abnormal in the presence of a circulating inhibitor to any of the intrinsic pathway factors. The differentiation of inhibitors from factor depletion is important and can best be accomplished by a mixing study in which patient and normal plasma are combined in a 1:1 ratio and the test is repeated on the mixed sample. If the abnormal value is corrected completely, the problem is factor deficiency. If the result does not change or the abnormality is corrected only partially, an inhibitor is present. This difference stems from the above mentioned fact that the aPTT will be normal in the presence of 50% normal activity.