Prothrombin Time (PT) Test

The Prothrombin Time (PT) test, also known as the protime test or PT/INR test, is a blood test used to measure the time it takes for blood to clot. It is primarily used to monitor patients on warfarin therapy, but it can also be used to diagnose bleeding disorders and liver disease.

What does the test measure?

The PT test measures the integrity of the extrinsic and common coagulation pathways, which involve factors I, II, V, VII, and X. The test is based on the principle that thromboplastin, a substance extracted from tissue, is added to a blood sample, and the time it takes for the blood to clot is measured.

How is the test performed?

The PT test is typically performed on a blood sample collected from a vein in the arm. The blood sample is then mixed with thromboplastin, and the mixture is incubated at 37°C (98.6°F) for a specified period. The time it takes for the blood to clot is measured using a coagulometer or a manual tilt tube method.

What are the results?

The PT test results are reported in seconds, and they can be expressed as:

• Prothrombin time (PT): The time it takes for the blood to clot, usually reported in seconds.
• International normalized ratio (INR): A calculated value that standardizes the PT result for different thromboplastin reagents and laboratories. INR is used to monitor patients on warfarin therapy.

Reference ranges

The reference range for PT varies depending on the laboratory and the thromboplastin reagent used, but a typical range is:

• PT: 11-14 seconds
• INR: 0.9-1.2

Clinical significance

Prolonged PT or elevated INR values may indicate:

• Bleeding disorders, such as hemophilia or vitamin K deficiency
• Liver disease, such as cirrhosis or hepatitis
• Warfarin therapy, which requires monitoring to prevent bleeding complications
• Vitamin K deficiency or malabsorption

Shortened PT values may indicate:

• Hypercoagulability, which increases the risk of thrombosis
• Cancer or other conditions that activate the coagulation cascade