PROTHROMBIN TIME TEST (PT/INR)
Overview
A prothrombin time (PT/INR) test measures how fast a blood sample forms a clot. Healthcare providers often perform this test to monitor how fast your blood clots if you’re taking the blood thinner warfarin. It’s also used to diagnose blood disorders. A high PT/INR means your body takes longer than normal to form blood clots.
What is a Prothrombin Time (PT/INR) Test?
A prothrombin time (PT or PT/INR) test measures how quickly your blood clots. Results may be given in seconds or, more commonly, as the International Normalized Ratio (INR). Prothrombin is a protein your liver makes, and it plays an essential role in the clotting process.
When is a PT Test Performed?
Your healthcare provider may perform a PT test to:
- Monitor how warfarin (Coumadin®) is working.
- Plan for surgery by ensuring your blood is clotting normally.
- Diagnose bleeding and blood clotting disorders.
- Check for liver conditions.
Test Details
How Does a Prothrombin Time Test Work?
A PT test is a simple blood test. After your blood is drawn, a substance called thromboplastin is added to activate clotting. A lab specialist or device then measures how long it takes for the blood sample to form a clot.
How Do I Prepare for a PT Test?
Your healthcare provider may ask you to:
- Avoid certain medications that can affect your results (e.g., some antibiotics).
- Cut back on vitamin K supplements or foods high in vitamin K (e.g., green tea, asparagus, broccoli), as these can impact your results.
Always check with your provider before stopping any medications or altering your diet.
What Should I Expect During a PT Test?
The process is like a standard blood draw. A healthcare provider will:
- Swab your skin with alcohol and place an elastic band around your upper arm.
- Ask you to make a fist to help blood flow.
- Insert a needle to collect blood into a vial.
- Bandage the spot where the needle was inserted.
Sometimes, a fingertip prick is used instead. There are also at-home test kits available for frequent testing, especially for people taking warfarin.
What Should I Expect After a PT Test?
You can usually go about your regular activities after the test. There may be slight discomfort or bruising where the needle was inserted, but this typically resolves quickly.
Results and Follow-Up
What Do the Results Mean?
PT/INR results are measured in seconds or as an INR value, which standardizes the result:
- Normal PT range: 11 to 13.5 seconds.
- Normal INR range: 0.8 to 1.1.
- If you’re taking warfarin, your INR should be between 2.0 to 3.0.
Abnormal PT Results
A high PT or INR result means it takes longer than usual for your blood to clot, which could indicate:
- A bleeding or blood clotting disorder.
- A liver problem.
- A deficiency of vitamin K.
If you’re taking warfarin, an abnormal result could mean:
- Your warfarin dose is too high or too low.
- You’ve taken medications or supplements that interact with warfarin.
- You’ve consumed foods or drinks that interact with warfarin, such as kale, spinach, cranberry juice, or alcohol.
When Should I Know the Results?
Results from a PT/INR test are usually available within a few hours to a day. Finger prick tests can produce results in minutes. If the sample is sent to an off-site lab, results may take several days.
What Happens If the Results Are Abnormal?
If your PT results are abnormal, your healthcare provider may order additional tests to determine the cause. If you're taking warfarin, your provider may adjust your dosage to ensure your blood is clotting properly without the risk of forming dangerous clots.