What Do Your PT, PTT, and INR Results Mean?

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Before you have surgery your doctor may order blood tests to determine how quickly your blood clots. This group of tests is known as a coagulation study. Individually these tests are commonly referred to as a prothrombin time (PT), partial thromboplastin time (PTT), and international normalized ratio (INR).

These pre-surgical tests determine whether your blood clots normally and are used to avoid excessive bleeding during surgery. Drugs used to slow clotting have a variety of names, but heparin, Coumadin (warfarin), and Lovenox (enoxaparin) are among the most common. These medications usually need to be stopped for a period of time prior to surgery.

Types of Coagulation Tests
Verywell / JR Bee

Normal Values for Coagulation Tests

There is a normal range of values seen in patients not taking blood thinners. These ranges differ from the values desired when a person is taking a blood thinner.

Normal Ranges

Normal values for people not on blood thinners:

  • PT: 10 to 12 seconds (this can vary slightly from lab to lab)
  • PTT: 30 to 45 seconds (this can value slightly from lab to lab)
  • INR: 1 to 2

Blood thinners will make blood take longer to clot, so a patient taking a blood thinner would be expected to have lab results that are higher (longer) than the ones listed here.

Prothrombin Time (PT)

This test is done to evaluate the blood for its ability to clot. It is often done before surgery to evaluate how likely the patient is to have a bleeding or clotting problem during or after surgery. Taking Coumadin (warfarin) is the most common cause of a prolonged PT.

Other possible causes include vitamin K deficiency, hormone drugs including hormone replacements and oral contraceptives, disseminated intravascular coagulation (a serious clotting problem that requires immediate intervention), liver disease, and the use of the anticoagulant drug warfarin.

Additionally, the PT result can be altered by a diet high in vitamin K, liver, green tea, dark green vegetables, and soybeans.

Partial Thromboplastin Time (PTT)

This test is performed primarily to determine if heparin (blood thinning) therapy is effective. It can also be used to detect the presence of a clotting disorder. It does not show the effects of drugs called “low molecular weight heparin” or most commonly by the brand name Lovenox.

Extended PTT times can be a result of anticoagulation therapy, liver problems, lupus, and other diseases that result in poor clotting.

International Normalized Ratio (INR)

The INR is used to make sure the results from a PT test is the same at one lab as it is at another lab. In the 1980s the World Health Organization determined that patients may be at risk because the results of a PT test would vary from one lab to another, based upon the way the test was done.

The “normal” range for one lab would be different than a “normal” value from another lab, creating problems for patients who were being treated in several locations. In order to standardize the results between labs, the INR was created. The INR result should be the same, regardless of the location where the tests are performed.

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