Let’s dive into prothrombin time (PT) and partial thromboplastin time (PTT) tests and discuss what PT can tell us about liver function.
The coagulation pathway is made up of the extrinsic, intrinsic, and common pathways. Prothrombin time is a coagulation test used to measure the integrity of the extrinsic and common coagulation pathways. Partial thromboplastin time measures the integrity of the intrinsic and common coagulation pathways.
What are the normal ranges for PTT and PT?
Partial thromboplastin time has a normal range of 25–37 seconds. As well, prothrombin time is expressed in seconds or as a ratio called the international normalized ratio (INR). Prothrombin time has a normal range of 9.4–12.5 seconds or 0.9–1.1 INR.
What does an abnormal PT tell us?
Prothrombin time involves the interactions of factors I (fibrinogen), II (prothrombin), V, VII, and X. These factors are all synthesized by the liver. Remember, the common coagulation pathway is measured by both PT and PTT, but the extrinsic pathway is only measured by PT. So, if PT is abnormal and PTT is normal, we know the problem lies with the extrinsic pathway.
The clotting factors involved in the extrinsic pathway are factors III and VII. Factor VII has the shortest half-life of all the clotting factors, which is 4–6 hours. Thus, when the liver is injured and there is compromised liver function, factor VII levels quickly fall. Within a short time after significant liver dysfunction, PT is prolonged because the liver cannot make enough blood-clotting proteins. Consequently, it takes longer for the blood to clot.
So, if a patient has a normal PTT and an abnormal PT, we know that the abnormality must lie with one of the extrinsic pathway factors (e.g., III or VII). Since factor III is one of the reagents added when performing the PT test (and a factor III deficiency has never been described), we can conclude that the patient has low levels of factor VII.
Low factor VII levels may be due to reduced factor synthesis or the presence of a factor VII inhibitor (which is rare). Causes of reduced synthesis of factor VII include congenital or genetic disorders, liver disease, and warfarin. Liver disease is usually the culprit since congenital causes are rare and warfarin is easily ruled out.
As you can see, PT is an important test for assessing liver function. You can review the coagulation pathways and learn how to interpret PT and PPT in more detail in our Hematology and Coagulation Essentials course.
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