A prothrombin time test is a blood test used to evaluate blood clotting. INR stands for international normalized ratio, a calculation that expresses the results of the time test.

Prothrombin is a protein made by the liver to assist in blood clotting. Blood clotting helps stop bleeding when you get a wound, but may also lead to blockage in an artery or vein. This can cause a stroke, heart failure, or heart attack. Your doctor may order a coagulation panel if they suspect you may have a condition that affects your body’s normal blood clotting functions. Conditions that may be diagnosed through a PT/ INR test include:

  • Hemophilia (a bleeding disorder that may result in spontaneous bleeding)
  • Thrombosis (blood clots causing a blockage in blood vessels)
  • Liver disease
  • Vitamin K deficiency

In addition to screening for these blood clotting conditions, a PT/INR test is commonly ordered for individuals who take blood-thinning medication such as warfarin. Warfarin is prescribed as a treatment for blood clots.

A PT/INR test is used to evaluate your body’s blood clotting function. This test is commonly ordered to:

  • Monitor the efficacy of warfarin - a medication that is prescribed to treat blood clots
  • Diagnose the cause of abnormal blood clotting
  • Diagnose the cause of any unusual bleeding
  • Evaluate your body’s blood clotting ability before surgery
  • Check for liver disease
  • Screen individuals waiting for liver transplants

Blood clot disorders often involve the body’s inability to properly clot blood, or excessive blood clotting that can lead to blockage in the veins and arteries.

Common symptoms of excessive blood clotting include:

  • Pain in the arms or legs
  • Swelling in the arms or legs
  • Redness in the affected area
  • Difficulty breathing
  • Chest pain
  • Rapid heart rate

Common symptoms of poor blood clotting include:

  • Unexplained nose bleeds
  • Unexplained heavy bleeding
  • Blood in the urine or stool
  • Fatigue
  • Joint pain caused by internal bleeding
  • Headaches
  • Heavy menstrual periods
  • Warmth in muscles or joints

If you are currently taking warfarin, you may be asked to suspend dosage until after the test. Aside from this, no specific preparation is needed for a PT/INR test, unless ordered otherwise by your doctor.

Most PT/INR tests are performed at a primary care office. A blood sample will be drawn from your body via a small puncture from a needle.

During your appointment, your arm will be wrapped in a band to push blood into the veins of the arm. This makes it easier to insert the needle that will be drawing blood from that area. You may feel a slight pinch as the needle is inserted into your arm, but this sensation should pass within a few seconds. The needle draws blood from a vein in your arm, depositing it into a vial or test tube. Some individuals or tests may require multiple tubes to be filled.

After the needed about of blood has been drawn, the needle will be taken out of your arm and the puncture site will be cleaned and bandaged. The blood will then be sent to a lab for testing and analysis.

You will get your results for most blood tests in 1-3 business days. Depending on the results of your blood test, your health care provider may request follow-up testing to provide a definitive diagnosis of any health conditions or diseases they may detect.

Individuals who are currently taking warfarin will receive their results through INR levels.

Low INR: A low INR ratio means that you may be at risk for dangerous blood clots.

Excessive INR: A high INR ratio (above 3.0) means that your blood clots too slowly, possibly increasing your risk of dangerous bleeding disorders.

Depending on your INR results, your doctor may choose to alter your warfarin prescription dosage.

For individuals who are not currently taking warfarin, results for a PT test are commonly given in seconds. The average time for healthy individuals’ blood to clot is about 10-13 seconds. If your results show that it took longer for your blood to clot, you may have a bleeding condition. If the results are below this average, it means that your blood clots more quickly than normal.

An abnormal clotting time may indicate a condition such as:

  • A vitamin K deficiency
  • Liver disease
  • Inadequate levels of proteins that help the blood to clot
  • Blood-thinning medication usage
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