How to interpret a partial thromboplastin time (PTT) test
Learn the mechanisms behind PTT tests and how to interpret the results.
Learn how to pinpoint the cause of prolonged PTT and investigate bleeding in patients with normal PTT. In this video, from our Hematology and Coagulation Essentials course, you'll learn all about the mechanisms behind PTT tests and how to interpret the results. You will be able to formulate a common differential diagnosis for prolonged PTT and know which tests to order next.
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[00:00:00] Here we're going to discuss the partial thromboplastin time or PTT. PTT is a test to measure the integrity of the intrinsic and the common pathway. To measure PTT, blood is collected in a citrate tube. Patient's platelet-poor plasma is mixed with a surface activating agent such as silica or kaolin and platelet substitute such as crude phospholipid is added. Clotting time is measured and the normal PTT is up to approximately
[00:00:30] 35 seconds. PTT measures the integrity of the intrinsic and the common pathway. The PT is a test for the integrity of the extrinsic and the common pathway. If an individual has isolated prolonged partial thromboplastin time, then the defect is in the intrinsic pathway. Within the intrinsic pathway, are factors 12, 11, 9, and 8, therefore, causes of an
[00:01:00] isolated prolonged partial thromboplastin time should be due to deficiency or inhibitors of these clotting factors. However, please know that if an individual has abnormality of factor 12, the patient does not develop any bleeding. If levels of factor 8, 9, 11 are reduced and this condition is inherited, these are known as hemophilia A, B, and C respectively. There are
[00:01:30] some other causes of isolated prolonged partial thromboplastin time and these include inhibitors to factor 8, 9, and 11, Von Willebrand's disease, the drug heparin, and lupus anticoagulant. Patients with von Willebrand disease have low levels of factor 8. Heparin and lupus anticoagulant acts against multiple clotting factors, thus, prolonging PTT. When faced with a prolonged PTT, what should
[00:02:00] we do next? The next test to order is a mixing study, which will be discussed again in another lesson.