Prothrombin Time (PT)/International Normalized Ratio (INR)

The prothrombin time (PT) and international normalized ratio (INR) are blood tests used to measure the time it takes for blood to clot and to evaluate the effectiveness of warfarin therapy.

What is Prothrombin Time (PT)?

Prothrombin time (PT) is a test that measures the time it takes for blood to clot. It is used to assess the extrinsic coagulation pathway, which involves factors VII, X, V, II (prothrombin), and fibrinogen. The PT test is typically reported in seconds.

What is International Normalized Ratio (INR)?

The international normalized ratio (INR) is a calculation based on the PT result that standardizes it for different laboratories and reagents used. INR takes into account the variability of thromboplastin, a substance used to initiate clotting in the PT test. The INR is calculated using the following formula: INR = (PT patient / PT normal) ^ ISI, where ISI is the international sensitivity index.

Purpose of PT/INR Testing

The primary purpose of PT/INR testing is to monitor patients on warfarin therapy. Warfarin works by inhibiting the production of vitamin K-dependent clotting factors (II, VII, IX, and X), which prolongs the PT and increases the INR. The goal of warfarin therapy is to maintain an INR within a therapeutic range, usually between 2.0 and 3.0, to prevent thrombosis while minimizing the risk of bleeding.

Interpretation of Results

The interpretation of PT/INR results depends on the clinical context:

• Normal PT/INR: A normal PT is typically between 11 and 14 seconds, and a normal INR is between 0.9 and 1.1.
• Prolonged PT/INR: A prolonged PT or elevated INR indicates an increased risk of bleeding and may be seen in patients on warfarin therapy, those with liver disease, or those with vitamin K deficiency.
• Shortened PT/INR: A shortened PT or decreased INR may indicate a hypercoagulable state, increasing the risk of thrombosis.

Clinical Applications

PT/INR testing has several clinical applications:

• Monitoring warfarin therapy: Regular PT/INR testing is essential to adjust warfarin doses and maintain a therapeutic INR range.
• Evaluating bleeding or thrombotic disorders: PT/INR tests can help diagnose coagulopathies, such as hemophilia or factor deficiencies.
• Assessing liver function: The PT/INR test can be used to evaluate liver synthetic function and detect liver disease.

Limitations and Precautions

PT/INR testing has some limitations and precautions:

• Vitamin K-dependent factors: PT/INR tests only measure the extrinsic coagulation pathway and do not assess the intrinsic pathway or other clotting factors.
• Thromboplastin variability: Different thromboplastins can affect PT results, which is why INR standardization is essential.
• Interference from other medications: Certain medications, such as antibiotics or antiplatelet agents, can interfere with warfarin therapy and PT/INR results.