Overview of PT
What is PT?
Why is PT done?
The Prothrombin Time Test is performed:
· To detect and diagnose bleeding disorders or clotting disorders
· Before the surgical procedure
· To monitor the efficacy of treatment with anticoagulating agent, Warfarin
What does PT Measure?
In case of any bleeding, the body responds to stop the blood loss as quickly as possible by forming a blood clot. This process of blood clotting or coagulation is called hemostasis and involves a series of chemical reactions in the blood (coagulation cascade) which activates blood proteins called coagulation factors one after another, in a series. The activated coagulation factors lead to the formation of fibrin mesh around the platelets and other blood cells at the site of bleeding and this complex hardens to form a “blood clot”.
Coagulation cascade proceeds by two pathways, the intrinsic pathway, and the extrinsic pathway. These pathways later merge together into a common pathway. Prothrombin (Coagulation Factor II) is converted to its active form thrombin in one of the reactions. The PT Test assesses the functioning of Blood Coagulation Factors I, II, V, VII, and X, which are parts of the extrinsic and common pathways by measuring the time taken for conversion of prothrombin to thrombin. The Activated Partial Thromboplastin Time (aPTT) Test measures the functioning of Blood coagulation Factors I, II, V, XII, VIII, IX, X, and XI, along with other factors Prekallikrein (PK), and High Molecular Weight Kininogen (HK) which form parts of the intrinsic and common coagulation pathways. The PT and aPTT tests are usually performed together and the results of both are evaluated simultaneously to determine the rate of blood clotting.
Warfarin is a drug prescribed in the treatment of conditions caused or worsened by excessive blood clotting like Deep Vein Thrombosis (blood clot formation in blood vessels), irregular heartbeats, etc. The PT test is also performed to evaluate the effectiveness of Warfarin treatment. When performed to determine Warfarin efficacy, the result of the PT Test is expressed in terms of a measurement scale called the International Normalized Ratio (INR). Since the PT Tests are performed using different reagents in different labs giving rise to widely varying results in other conventional units, the INR system was created to maintain uniformity in the results.
Preparation for PT
- No special preparation required
Sample Type for PT
Interpreting PT results
Normal Reference Ranges:
Prothrombin time (PT): 8.7-11.5 seconds
International normalized ratio (INR): 0.8-1.2
INR during Warfarin treatment: 2.0 to 3.0
Prothrombin time longer than the normal range indicates a bleeding disorder where blood takes a longer time to clot.
Prothrombin time shorter than the normal range indicates a clotting disorder where blood takes lesser time to clot.
The PT and aPTT tests are usually performed and evaluated simultaneously, and the results may be interpreted as follows:
Liver diseases, Vitamin K deficiency, reduced activity of Factor VII, Disseminated Intravascular Coagulation (DIC), Warfarin treatment
Reduced activity of Factors VIII, IX, XI, or XII, Von Willebrand disease, autoantibodies destroying one or more coagulation factors
Reduced activity of Factors I, II, V or X, liver diseases, DIC, an overdose of Warfarin
Normal hemostasis usually. However, normal results may also appear in a mild reduction in coagulation factor activity and mild cases of Von Willebrand disease.
Answers to Patient Concerns & Frequently Asked Questions (FAQs) about PT
Frequently Asked Questions about Prothrombin Time