What Do Your PT/INR and PTT Results Mean?

Blood tests used to assess blood clotting and clotting disorders

Prothrombin time (PT) and partial thromboplastin time (PTT) are tests used to evaluate coagulation, or the clotting of blood. While both measure how quickly blood clots, they have distinct purposes:

  • The PT test, also known as the PT/INR test, looks at the extrinsic pathway of coagulation (meaning coagulation that occurs after blood escapes a blood vessel).
  • The PTT test looks at the intrinsic pathway of coagulation (meaning coagulation that occurs within a blood vessel).

When looked at individually and together, the PT and PTT can reveal a bleeding disorder and your risk for severe blood clots. It can also show how well your blood clots in advance of surgery and how well you are responding to anticoagulant therapy ("blood thinners").

This article explains how the PT and PTT tests work, what they indicate, and how the test results are interpreted.

Types of Coagulation Tests
Verywell / JR Bee

Prothrombin Time (PT)

The prothrombin time (PT) test measures how long it takes for a blood clot to form based on a protein produced by the liver called prothrombin.

Prothrombin, also known as clotting factor 2, is one of 13 substances known as "clotting factors" that are involved in coagulation. How fast blood clots after bleeding depends on how much of each clotting factor is in the blood.

Based on the clotting time of the PT test, a lab may be able to determine:

  • If you have a clotting disorder like deep venous thrombosis (DVT)
  • If you have a bleeding disorder like hemophilia
  • If you are at risk of excessive bleeding when undergoing surgery
  • How well a specific anticoagulant known as warfarin is working
  • If you have any liver problems

How the Test Works

Once blood is drawn, a substance called a tissue factor is added to the test tube. Tissue factor, also known as clotting factor 3, activates the sample in a way that illustrates how blood would clot if there is bleeding (the extrinsic pathway).

Once activated, the PT test is measured in seconds. The time is then compared to the reference range of values (meaning the time span in which blood clotting is considered normal).

With the PT test, the reference range is between 11 to 13.5 seconds if you are not on anticoagulants.

A number higher than the reference range means your blood is taking longer than usual to clot. A number lower than the reference range means that your blood is clotting faster than normal. There are many possible reasons why these might occur.

One of the key reasons is that you may have too much or too little vitamin K in your blood, which your body uses to build clotting factors.

Other medical conditions, substances, and diseases can also affect normal blood clotting.

Clotting Too Quickly
Clotting Too Slowly

The PT test does not require fasting but foods high in vitamin K can skew the results, including beef or pork liver, green tea, dark green vegetables, or soybeans.

International Normalized Ratio (INR)

The PT test is sometimes referred to as the PT/INR test. The INR refers to the international normalized ratio, a calculation that helps ensure test results are standardized from one lab to the next.

Because there are variations in the different types of tissue factor used for the PT test, the INR calculation ensures that the clotting time is accurate based on the body of test results from tissue factor manufacturers worldwide.

INR values are important because they help determine how well a person is responding to warfarin, one of the most commonly prescribed anticoagulants used to prevent blood clots.

For people on warfarin, the reference range of values for PT/INR is 2 to 3 seconds.

High and low INR values are interpreted as follows:

  • Low INR values mean that you may be at risk of dangerous blood clots.
  • High INR values mean that you may be at risk of dangerous bleeding.

Based on the values, a healthcare provider can decide if the warfarin dose needs to be adjusted or if other interventions are needed.

Partial Thromboplastin Time (PTT)

The partial thromboplastin time (PTT) test also measures the speed of clotting but differs from the PT test in that it aims to establish how blood clots within a blood vessel (intrinsic pathway). This is based in part on an enzyme called thromboplastin (clotting factor 11) that converts prothrombin into its more active form, called thrombin.

In looking at how clotting works within a blood vessel, the PTT test can help determine:

  • Why you are experiencing abnormal bruising or bleeding
  • If you have an autoimmune disease like lupus or antiphospholipid syndrome (APS) that causes blood clots
  • The risk of bleeding before surgery in people with bleeding disorders
  • Your response to a specific anticoagulant called heparin

How It Works

The processes involved in the PTT test are similar to the PT test, but, instead of using tissue factor, the blood sample is activated with minerals or acids that show how blood clotting occurs when something is introduced into the blood.

With the PTT test, the reference range is between 25 and 33 seconds.

As with the PT test, a higher PTT number means your blood is taking longer than usual to clot. A lower PTT number means that your blood is clotting faster than normal. There are several reasons why this might occur.

Clotting Too Quickly
  • An advanced whole-body infection that is using up clotting factor too quickly

  • Metastatic ovary, colon, or pancreatic cancer

Clotting Too Slowly
  • Vitamin K deficiency

  • Too much heparin

  • Liver disease

  • Hemophilia

  • Von Wildebrand disease

  • Lupus

  • Antiphospholipid syndrome (APS)

  • Certain types of leukemia

The PTT is especially important in determining a person's response to heparin. Unlike warfarin, which is taken by mouth to prevent clots, heparin is given by injection for people who have undergone surgery or are at risk of blood clots.

Heparin also differs in that interferes with the action of thromboplastin, whereas warfarin interferes with the action of vitamin K.

By understanding the nature of intrinsic blood clotting, a surgeon can determine how much heparin is needed for surgery for each individual patient.

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).


The prothrombin time (PT) test and partial thromboplastin time (PTT) measure how quickly blood clots. The tests can help detect bleeding disorders, check a person’s response to blood thinners, and assess a person’s risk for bleeding prior to surgery.

The PT test is useful in evaluating the effects of the anticoagulant warfarin and identifying clotting and bleeding disorders. The international normalized ratio (INR) test ensures that PT results are standardized.

The PTT is mainly used to monitor a person’s response to the anticoagulant heparin and can also identify diseases that interfere with blood clotting.

2 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. Prothrombin time test and INR (PT/INR).

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

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.