What Do Your PT, PTT & INR Results Mean?

Understanding blood clot test results

Scientist examining test tubes in lab
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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 PT (Prothrombin Time), PTT (Partial Thromboplastin Time), and INR (International Normalized Ratio).

During some surgeries it is important that the blood not clot as quickly as normal, and medications may be given to slow the clotting time.

Drugs commonly used to slow clotting have a variety of names, but Heparin, Coumadin, Lovenox, and Warfarin are among the most common. In other cases, the patient may not clot quickly enough, and steps may be taken to make the blood clot more quickly.

The results listed below are "normal values" rather than the results we are hoping for when an individual is taking a blood thinner.  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 Blood Test-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.

Normal PT Values: 10-12 seconds (this can vary slightly from lab to lab)

Common causes of a prolonged PT 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 anti-coagulant 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 Blood Test-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.

Normal PTT Values: 30 to 45 seconds (this can value slightly from lab to lab)

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

International Normalized Ratio Blood Test-INR

Normal INR Values: 1 to 2

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.


PTT. The American Association of Clinical Chemistry https://labtestsonline.org/understanding/analytes/aptt/tab/glance

Zehnder JL. Clinical use of coagulation tests. In: UpToDate. 2017. http://www.uptodate.com/contents/clinical-use-of-coagulation-tests