Prothrombin (PT/INR)

Prothrombin Time (PT/INR) Testing

Prothrombin Time (PT) and International Normalized Ratio (INR) are important blood tests used to assess your blood's clotting ability. These tests measure how long it takes for a sample of your blood to form a clot.

Why is this test performed?

• Monitoring Warfarin (Coumadin) Therapy: The most common use for this test is to monitor the effectiveness of anticoagulant medications, such as warfarin. Regular PT/INR testing helps healthcare providers adjust the dose to ensure the blood is thin enough to prevent dangerous clots but not so thin that it causes excessive bleeding.
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• Diagnosing Bleeding or Clotting Disorders: The test can help identify the cause of unexplained bleeding or a tendency to clot too easily.
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• Assessing Liver Function: The liver produces many of the proteins (clotting factors) necessary for blood clotting. An abnormal PT/INR can be an indicator of liver disease, as a damaged liver may not produce enough of these factors.
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• Pre-Surgical Screening: PT/INR tests are often performed before surgery to ensure a patient's blood will clot normally, reducing the risk of excessive bleeding during the procedure.
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How It Works

A small blood sample is drawn, typically from a vein in your arm. In the laboratory, a reagent is added to the blood, and the time it takes for a clot to form is measured.

Understanding the Results

• Prothrombin Time (PT): This measures the time in seconds. A normal PT for someone not on blood thinners is typically around 10-13 seconds.
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• International Normalized Ratio (INR): The INR is a standardized calculation derived from the PT result. It was developed to allow for consistent comparison of results regardless of the lab or the testing method used. This is especially crucial for patients on warfarin.
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• Normal INR: For healthy individuals not on blood thinners, a normal INR is typically around 1.1 or below.
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• Therapeutic Range: For patients on warfarin, the goal is to have a higher INR, usually between 2.0 and 3.0, to prevent clots. The specific target range can vary depending on the medical condition being treated.
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What to Do with Your Results

Your healthcare provider will interpret your results in the context of your medical history and other factors. They will use this information to:

• Adjust your medication dosage.
• Investigate potential underlying medical conditions.
• Determine if further testing is necessary.