Proceeds from website advertising help sustain Lab Tests Online. AACC is a not-for-profit organization and does not endorse non-AACC products and services.

PT and INR

Print this article
Share this page:
Also known as: Prothrombin Time; Pro Time; Protime
Formal name: Prothrombin Time and International Normalized Ratio

At a Glance

Why Get Tested?

To check how well the blood-thinning medication (anticoagulant) warfarin (COUMADIN®) is working to prevent blood clots; to help detect and diagnose a bleeding disorder

When to Get Tested?

When you are taking warfarin or when your doctor suspects that you may have a bleeding disorder

Sample Required?

A blood sample drawn from a vein in the arm; sometimes blood from a fingerstick

Test Preparation Needed?

None needed, although if you are receiving anticoagulant therapy, the specimen should be collected before taking your daily dose.

The Test Sample

What is being tested?

The prothrombin time (PT) test measures how long it takes for a clot to form in a sample of blood. In the body, the clotting process involves a series of sequential chemical reactions called the coagulation cascade, in which coagulation or "clotting" factors are activated one after another and result in the formation of a clot. There must be a sufficient quantity of each coagulation factor, and each must function properly, in order for normal clotting to occur. Too little can lead to excessive bleeding; too much may lead to excessive clotting.

In a test tube, there are two "pathways" that can initiate clotting, the so-called extrinsic and intrinsic pathways. Both of these then merge into a common pathway (like the shape of a "Y") to complete the clotting process. In one of the final steps of the clotting cascade, prothrombin (also called Factor II) is converted into thrombin, but this factor and step is not the sole focus of the PT test.

The PT test evaluates how well all of the coagulation factors in the extrinsic and common pathways of the coagulation cascade work together. Included are: Factors I (Fibrinogen), II (Prothrombin), V, VII and X. The PT test evaluates the overall ability to produce a clot in a reasonable amount of time and, if any of these factors are deficient or dysfunctional, the PT will be prolonged.

The PT test is usually measured in seconds and is compared to a normal range that reflects PT values in healthy individuals. Because the reagents used to perform the PT test vary from one laboratory to another and even within the same laboratory over time, the normal ranges also will fluctuate. To standardize results across the U.S. and the world, a World Health Organization (WHO) committee developed and recommended the use of the Internationalized Normalized Ratio (INR) with the PT test for people who are receiving the anticoagulant warfarin (COUMADIN®).

The INR is a calculation that adjusts for changes in the PT reagents and allows for results from different laboratories to be compared. Most laboratories are now reporting both PT and INR values whenever a PT test is performed. The INR is only applicable, however, for those taking the blood-thinning medication warfarin.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein in the arm or, sometimes, from a fingerstick.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed. If a person is receiving anticoagulant therapy, the specimen should be collected before the daily dose is taken.

The Test

Common Questions

Ask a Laboratory Scientist

Form temporarily unavailable

Due to a dramatic increase in the number of questions submitted to the volunteer laboratory scientists who respond to our users, we have had to limit the number of questions that can be submitted each day. Unfortunately, we have reached that limit today and are unable to accept your inquiry now. We understand that your questions are vital to your health and peace of mind, and recommend instead that you speak with your doctor or another healthcare professional. We apologize for this inconvenience.

This was not an easy step for us to take, as the volunteers on the response team are dedicated to the work they do and are often inspired by the help they can provide. We are actively seeking to expand our capability so that we can again accept and answer all user questions. We will accept and respond to the same limited number of questions tomorrow, but expect to resume the service, 24/7, as soon as possible.

Article Sources

« Return to Related Pages

NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 807-811.

Clarke, W., Editor (© 2011). Contemporary Practice in Clinical Chemistry 2nd Edition: AACC Press, Washington, DC. Pp 265-281.

Brooks, M. (2012 February 3). Close INR Monitoring Needed When Warfarin Users Add an Antibiotic. Medscape Today News from Reuters Health Information [On-line information]. Available online at through Accessed May 2012.

Martin, C. and Beardsell, I. (2012 March 8). Is Routine Coagulation Testing Necessary in Patients Presenting to the Emergency Department With Chest Pain? Medscape News Today from Emerg Med J. 2012;29(3):184-187 [On-line information]. Available online at through Accessed May 2012.

Marlar, R. and Gausman, J. (2011 February 23). Do You Report an Accurate International Normalized Ratio? Find Out Using Local Verification and Calibration. Medscape Today News from Lab Med. 2011;42(3):176-181 [On-line information]. Available online at through Accessed May 2012.

Dugdale, D. (Updated 2011 February 13). Prothrombin time (PT). MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed May 2012.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber’s Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby’s Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

(2001). International Normalized Ratio (INR). University Pathology Consortium [On-line handbook]. Available online at through

(2000-2004) INR. Northwest Cardiovascular Associates, S.C. [On-line information]. Available online at through

Brose, M. (2003 June 1). Prothrombin time (PT). MedlinePlus Medical Encyclopedia [On-line information]. Available online at

Brooke Huffman, G. (2001 April 15). Management of Patients Taking Warfarin. American Family Physician, Tips from other Journals [On-line journal]. Available online at through

Walling, A. (2003 February 15). Optimal Anticoagulation: Determining the Safest INR. American Family Physician, Tips from other Journals [On-line journal]. Available online at through

Miller, K (2003 September 15). Warfarin Management with Mild Elevation of INR. American Family Physician, Tips from other Journals [On-line journal]. Available online at through

Sadovsky, R. (2002 February 15). Factors Affecting the Delay in Return of Therapeutic INR Level. American Family Physician, Tips from other Journals [On-line journal]. Available online at through

Horton, J. and Bushwick, B. (1999 February 1). Warfarin Therapy: Evolving Strategies in Anticoagulation. American Family Physician, Clinical Pharmacology [On-line journal]. Available online at through

Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007, pp 733-737.

Pagana K, Pagana T. Mosby's Manual of Diagnostic and Laboratory Tests. 3rd Edition, St. Louis: Mosby Elsevier; 2006, pp 441-443.

Levin, M. (Updated March 9, 2007) Prothrombin Time (PT). MedlinePlus Encyclopedia. Available online at Accessed August 2008.

(Updated May 29, 2007) Prothrombin Time. Massachusetts General Hospital, Pathology Service. Available online at through Accessed August 2008.

(June 2008) Ansell J, et al. Pharmacology and management of the vitamin K antagonists: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). Chest. 2008 Jun;133(6 Suppl):160S-198S. Available online through Accessed August 2008.

Heart Rhythm Society. Patient Information, International Normalized Ratio. PDF available for download at through Accessed August 2008.

University of Alabama at Birmingham Coagulation Service. Bleeding Guidelines. Available online at Accessed August 2008.