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Activated Partial Thromboplastin Time Testing

written by: Teresa Martin•edited by: Emma Lloyd•updated: 7/11/2011

Formation of clots can be deadly, as can the treatment of them. Heparin therapy is used by many physicians to monitor the effects of the anticoagulant. To ensure proper medication activated partial thromboplastin time testing is performed in the laboratory.

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    The activated partial thromboplastin time testing is ordered during heparin therapy and in cases of bleeding disorders. Normal blood clotting is a physiological response when bleeding occurs. The first line of defense is the activation of platelets followed by a network of fibrin. Together platelets and fibrin form a mesh that are an important part of hemostasis. Normal coagulation takes place in two phases. The primary phase involves platelets and fibrinogen. The secondary phase of coagulation involves the activation of coagulation factors, also known as clotting factors. These clotting factors are a sequential series of proteins that aid in forming the fibrin plug.

    By using the activated partial thromboplastin time testing will demonstrate the presence, absence or deficiency of particular clotting factors.

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    Heparin Therapy Used to Resolve Clotting Problems

    Heparin therapy is initiated in cases of deep venous thrombosis, as well as other vascular disorders involving clotting problems. This therapy must be monitored very closely. Too much heparin will result in uncontrolled bleeding. Too small a dose can result in the continued formation of clots. Activated partial thromboplastin time testing is routinely used to monitor heparin therapy. Heparin works by increasing the activity of anti-thrombin to inhibit activated factors II, IX, X, XI, and XII.

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    Genetics and Bleeding Disorders

    Bleeding disorders are often the result of deficiencies in clotting factors which may be severe or mild and can be genetically related as in Hemophilia A, which is a deficiency of factor VIII. Disfunction of clotting capability can develop from liver disease. In this case the liver can no longer produce adequate amount of one or more of the factors. Other difficulties may involve Vitamin K which is needed for the liver to produce many of the factors. The body can also develop antibodies against some of the factors. Because of the wide variety of factors that may be involved in clotting disorders, coagulation studies are very important for diagnostic purposes.

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    How the Activated Partial Thromboplastin Time Test is Performed

    Testing the activated partial thromboplastin time involves drawing blood from a vein and mixing with an anticoagulant to prevent clotting. The sample is then mixed with calcium and the time (in seconds) it takes for a clot to form is the result. Normal ranges are between 25-39 seconds for the formation of a clot. Heparin therapy will lengthen the time of clotting and those being treated with heparin will have longer (therapeutic) clotting times.

    An elevated activated partial thromboplastin time test means there is a deficiency in one of the clotting factors. These clotting factors that are detectable by this test include I, II, III, IV, V, VI, VIII, IX, X, XI, and XII. In the absence of heparin therapy or other anticoagulant, an elevated activated partial thromboplastin time test result should be followed up by mixing studies. Mixing studies uses pooled plasma containing normal amounts of clotting factors added to the deficient sample. If the test results are normalized with the normal pooled plasma, then further testing to identify the specific clotting factor deficiencies may be done.