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

Tests Used to Evaluate Blood Clotting

Coagulation studies are used to determine how well your blood clots. This evaluation may involve prothrombin time (PT) and partial thromboplastin time (PTT) tests, as well as a calculation called the international normalized ratio (INR).

You may need one or more of these to monitor an illness, track the effects of blood thinners, or assess the risk of bleeding prior to surgery.

This article explains what coagulation is as well as the purpose of each of these tests, including what they can reveal about your health.

Types of Coagulation Tests
Verywell / JR Bee

What Is Coagulation?

Coagulation, also known as clotting, is the process in which blood turns from a liquid to a gel to form a clot.

It does so when red blood cells called platelets (thrombocytes) stick together. These cells are activated whenever the lining of blood vessels, known as the endothelium, is damaged or ruptures.

Coagulation is a normal process that prevents excessive bleeding, but there are times when the clotting process is abnormal and can cause harm.

On one hand, there are bleeding disorders like hemophilia that impair clotting and can lead to excessive bleeding. These conditions may require clot-promoting drugs like Tisseel (aprotinin) or compounds called clotting factors that aid with coagulation.

On the other hand, blood clots can form abnormally due to poor blood circulation. This could lead to the complete blockage of an artery in the lungs (pulmonary embolism), brain (stroke), or heart (heart attack). Blood thinners (anticoagulants) like heparin, Coumadin (warfarin), or Plavix (clopidogrel) are often prescribed for people who are at risk.


Coagulation involves the binding of red blood cells, called platelets, to form a clot. Certain bleeding disorders like hemophilia can impair blood clotting, while poor blood circulation can lead to the abnormal formation of a clot.

Purpose of Testing

Coagulation studies are used to evaluate blood clotting based on how fast clotting occurs in a sample of blood.

All that is required is a simple blood draw. Prior to the test, your healthcare provider may ask you to stop taking certain medications that can affect the results.

The tests can identify clotting problems and measure your response to therapies such as blood thinners or clotting factors. They are also commonly used before surgery to assess a person’s risk of bleeding.

Clotting problems are detected based on a reference range of values. Anything between the upper and lower values is considered normal. Anything outside of the upper or lower value is considered abnormal. The specific reference range can vary from one laboratory to another.


Coagulation studies are used to detect blood clotting disorders, monitor a person’s response to anti-clotting or pro-clotting therapies, or establish a person’s risk of bleeding prior to surgery.

Types of Test

Two of the tests used in a coagulation study—prothrombin time (PT) and partial thromboplastin time (PTT, also known as aPTT)—can reveal specific things about your health.

The third, called the international normalized ratio (INR), is technically a calculation more than it is a test. It is used to assess your response to blood thinners based on the PT.

Prothrombin Time (PT)

The prothrombin time (PT) test measures how quickly your blood clots. Taking the blood thinner warfarin is the most common cause of a prolonged PT. Generally, the reference range is 10–13 seconds, though it may vary.

Other possible causes include:

  • Vitamin K deficiency
  • Clotting factor deficiency
  • Liver disease
  • Hormonal therapies, including oral contraceptives
  • Disseminated intravascular coagulation (DIC), a rare but serious clotting disorder

The PT result can also be affected by a diet high in beef or pork liver, green tea, dark green vegetables, or soybeans.

Partial Thromboplastin Time (PTT)

The partial thromboplastin time (PTT) test also measures the speed of clotting but is primarily used to determine if heparin therapy is working. It can also help detect bleeding disorders.

Several medications and medical conditions can affect PTT results, including:

  • Warfarin
  • Vitamin C
  • Antihistamines
  • Aspirin
  • Anticoagulation therapy, including vitamin K antagonists (VKAs)
  • Thorazine (chlorpromazine)
  • Vitamin K deficiency
  • Liver problems
  • Lupus anticoagulant
  • Antiphospholipid syndrome

International Normalized Ratio (INR)

The international normalized ratio (INR) is a calculation derived from the PT test that helps ensure that test results are standardized from one lab to the next. For people on anticoagulant therapies such as warfarin, the INR should generally be around 2–3, though it might be higher for people at increased risk of clotting.


The prothrombin time (PT) test measures how quickly blood clots. The partial thromboplastin time (PTT) is mainly used to monitor a person’s response to anticoagulant therapies. The international normalized ratio (INR) calculation helps ensure that PT test results are standardized and accurate.

Risks and Complications

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

  • Injection site pain
  • Localized bruising
  • A blood-filled bump (hematoma) at the injection site
  • Lightheadedness or fainting
  • Excessive bleeding (especially if you’re taking blood thinners)
  • Infection (uncommon)

If you feel lightheaded during or after a blood draw, let one of the medical staff know.

Keep the injection site clean, and call your doctor if you have signs of infection (including fever, chills, or increasing pain, redness, warmth, or swelling at the injection site).


Coagulation studies involve a simple blood draw. A blood draw poses few risks but may cause injection site pain, bruising, and lightheadedness. Infection is rare.


Coagulation studies involve one or more blood tests that measure how quickly blood clots. The tests can help detect bleeding disorders, check a person’s response to anti-clotting or pro-clotting therapies, and assess a person’s risk for bleeding prior to surgery. Coagulation studies require a simple blood draw.

The prothrombin time (PT) test measures the rate of blood clotting in seconds and the international normalized ratio (INR) test ensures that PT results are standardized.

The partial thromboplastin time (PTT) is mainly used to monitor a person’s response to blood thinners.

Frequently Asked Questions

  • What happens if a PT test result is abnormal?

    Additional tests may be ordered if your PT results are abnormal. If you are not on blood thinners or clotting factors, you may need to start therapy to normalize PT levels. If you are on treatment, the dose may need to be adjusted.

  • Can your diet impact your INR results?

    Yes. If you are taking warfarin, certain foods that contain vitamin K, like dark leafy greens or green tea, may reduce its effectiveness. On the flip side, alcohol, cranberries, and grapefruit may increase the risk of bleeding by enhancing the effects of warfarin.

6 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. MedlinePlus. Partial thromboplastin time (PTT) test.

  2. Testing.com. Partial thromboplastin time (PTT, aPTT).

  3. Winter WE, Flax SD, Harris NS. Coagulation testing in the core laboratory. Lab Med. 2017;48(4):295-313. doi:10.1093/labmed/lmx050

  4. MedlinePlus. Prothrombin time (PT).

  5. Testing.com. Prothrombin time and international normalized ratio (PT/INR).

  6. Norwood DA, Parke CK, Rappa LR. A comprehensive review of potential warfarin-fruit interactions. J Pharm Pract. 2015;28(6):561-571. doi:10.1177/0897190014544823

By Jennifer Whitlock, RN, MSN, FN
Jennifer Whitlock, RN, MSN, FNP-C, is a board-certified family nurse practitioner. She has experience in primary care and hospital medicine.