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

And how can you change them?

Table of Contents
View All
Table of Contents

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).

Types of Coagulation Tests
Verywell / JR Bee

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.

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.

TEST NORMAL RANGES*
PT 10-12 seconds
PTT  30-45 seconds
INR  1:2 ratio
*For people not on blood thinners

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)

The prothrombin time (PT) test measures how quickly your blood clots. Taking Coumadin is the most common cause of a prolonged PT. Other possible causes include:

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

Partial Thromboplastin Time (PTT)

The partial thromboplastin time (PTT) test is performed primarily to determine if heparin (blood thinning) therapy is effective but also can detect a clotting/bleeding disorder. However, it can't be used to monitor the effect of drugs such as Lovenox, which are called “low molecular weight heparin” and used to prevent and treat blood clots in the legs and complications of heart disease.

Several conditions and medications can impact your PTT test results, including:

Ask your doctor if you should quit taking any of those drugs before the test. Do not go off of any medications without your doctor's advice.

International Normalized Ratio (INR)

The international normalized ratio (INR) is used to make sure the results from a PT test are the same from one lab to another. In the 1980s, the World Health Organization determined that people undergoing surgery may be at risk of excessive bleeding because different labs had different "normal" values than other labs. The INR result should be the same, regardless of the location where the tests are performed.

Associated Risks and Complications

Blood draws are considered routine, low-risk procedures. While rare, it is possible to have complications from a simple blood draw. These include:

  • Pain at the needle-insertion site
  • Bruising at the needle-insertion site
  • Lightheadedness or feeling faint
  • Anxiety over needles
  • Excessive bleeding, especially if you're taking blood thinners
  • Infection

If you feel lightheaded during or after a blood draw or the site continues to bleed, let someone at the facility know so they can assist or monitor you as needed. Watch for signs of infection (pain, redness, heat, swelling, fever, chills) over the next couple of days and get medical help if one develops.

Was this page helpful?
Article Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Tripodi A, Lippi G, Plebani M. How to report results of prothrombin and activated partial thromboplastin times. Clin Chem Lab Med. 2016;54(2):215-222. doi:10.1515/cclm-2015-0657

  2. National Institutes of Health, U.S. National Library of Medicine: MedlinePlus. Prothrombin time (PT). Updated April 2, 2021.

  3. Cleveland Clinic. Blood clotting disorders (hypercoagulable states). Updated April 25, 2019.

  4. Harvard College, T.H. Chan School of Public Health, The Nutrition Source. Vitamin K.

  5. National Institutes of Health, U.S. National Library of Medicine: MedlinePlus. Partial thromboplastin time (PTT) test. Updated December 10, 2020.

  6. National Institutes of Health, U.S. National Library of Medicine: MedlinePlus. Enoxaparin injection. March 17, 2021.

  7. Schreiber K, Sciascia S, de Groot PG, et al. Antiphospholipid syndromeNat Rev Dis Primers. 2018;4:18005. Published 2018 Jan 25. doi:10.1038/nrdp.2018.5

  8. St. Luke's Hospital: Multimedia Encyclopedia. Partial thromboplastin time (PTT). Updated January 27, 2015.

  9. University of Rochester Medical Center: Health Encyclopedia. International normalized ratio.

  10. National Institutes of Health, National Heart, Lung, and Blood Institute. Blood tests.

  11. KidsHealth from Nemours. Blood culture. Reviewed December 2017.

  12. University of Rochester Medical Center. Blood culture.