Prothrombin Time and International Normalized Ratio (PT/INR)
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- Also Known As:
- PT
- Pro Time
- Protime
- INR

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At a Glance
Why Get Tested?
A prothrombin time (PT) is a test used to help detect and diagnose a bleeding disorder or excessive clotting disorder; the international normalized ratio (INR) is calculated from a PT result and is used to monitor how well the blood-thinning medication (anticoagulant) warfarin (Coumadin®) is working to prevent blood clots.
When To Get Tested?
When you are taking warfarin or when you have unexplained or prolonged bleeding or inappropriate blood clotting
Sample Required?
A blood sample drawn from a vein; sometimes blood from a fingerstick
Test Preparation Needed?
None needed, although if you are receiving anticoagulant therapy, the blood sample should be collected before taking your daily dose.
What is being tested?
The prothrombin time (PT) is a test that helps evaluate your ability to appropriately form blood clots. The international normalized ratio or INR is a calculation based on results of a PT that is used to monitor individuals who are being treated with the blood-thinning medication (anticoagulant) warfarin (Coumadin®).
A PT measures the number of seconds it takes for a clot to form in your sample of blood after substances (reagents) are added. The PT is often performed along with a partial thromboplastin time (PTT) and together they assess the amount and function of proteins called coagulation factors that are an important part of proper blood clot formation.
In the body, when there is an injury and bleeding occurs, the clotting process called hemostasis begins. This process involves in part 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 or bleeding disorder; too much may lead to excessive clotting.
In a test tube during a laboratory test, there are two “pathways” that can initiate clotting, the so-called extrinsic and intrinsic pathways. Both of these then merge into a common pathway to complete the clotting process.
- 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/INR may be done at the same time as a PTT, which evaluates the clotting factors that are part of the intrinsic and common pathways: XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK).
The PT and PTT evaluate the overall ability to produce a clot in a reasonable amount of time and, if any of these factors are deficient in quantity or not functioning properly, the test results will be prolonged.
The PT 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 different laboratories in the U.S. and the world, a World Health Organization (WHO) committee developed and recommended the use of the Internationalized Normalized Ratio (INR), calculated based on the PT test result, for people who are receiving the anticoagulant warfarin (Coumadin®). Warfarin is prescribed for people with a variety of conditions, such as deep vein thrombosis (DVT) and some cardiovascular diseases (CVD) like atrial fibrillation to “thin” their blood and prevent inappropriate clotting.
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 report both PT and INR values whenever a PT test is performed. The INR should be only applicable, however, for those taking the blood-thinning medication warfarin.
Common Questions
Health Professionals – LOINC
LOINC Observation Identifiers Names and Codes (LOINC®) is the international standard for identifying health measurements, observations, and documents. It provides a common language to unambiguously identify things you can measure or observe that enables the exchange and aggregation of clinical results for care delivery, outcomes management, and research. Learn More.
Listed in the table below are the LOINC with links to the LOINC detail pages. Please note when you click on the hyperlinked code, you are leaving Testing.com and accessing Loinc.org.
LOINC | LOINC Display Name |
---|---|
52129-4 | INR post heparin adsorption Coag (PPP) [Relative time] |
34714-6 | INR Coag (Bld) [Relative time] |
46418-0 | INR Coag (BldC) [Relative time] |
38875-1 | INR Coag (Platelet poor plasma or blood) [Relative time] |
6301-6 | INR Coag (PPP) [Relative time] |
5964-2 | PT Coag (Bld) [Time] |
46417-2 | PT Coag (BldC) [Time] |
5901-4 | PT Coag (PPP control) [Time] |
42638-7 | PT Coagulation 1:1 saline (PPP) [Time] |
5902-2 | PT Coag (PPP) [Time] |
View Sources
Sources Used in Current Review
Prothrombin Time Test. Mayo Clinic. Available online at https://www.mayoclinic.org/tests-procedures/prothrombin-time/about/pac-20384661. Accessed on 9/18/18.
Prothrombin Time (PT) Test. Cleveland Clinic. Available online at https://my.clevelandclinic.org/health/diagnostics/17691-prothrombin-time-pt-test/test-details. Accessed on 9/18/18.
Prothrombin Time, Plasma. Mayo Medical Laboratories. Available online at https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/40934. Accessed on 9/19/18.
Prothrombin Time (PT). MedlinePlus. Available online at https://medlineplus.gov/ency/article/003652.htm. Accessed on 9/19/18.
(November 21, 2014) Prothrombin Time. Medscape. Available online at https://emedicine.medscape.com/article/2086058-overview. Accessed on 10/3/18.
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 http://www.medicine.uiowa.edu/path_handbook/upcmd/01025/inr.html.
(2000-2004) INR. Northwest Cardiovascular Associates, S.C. [On-line information]. Available online at http://www.nwcardio.com.
Brose, M. (2003 June 1). Prothrombin time (PT). MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003652.htm
Brooke Huffman, G. (2001 April 15). Management of Patients Taking Warfarin. American Family Physician, Tips from other Journals [On-line journal]. Available online at http://www.aafp.org/afp/20010415/tips/5.html.
Walling, A. (2003 February 15). Optimal Anticoagulation: Determining the Safest INR. American Family Physician, Tips from other Journals [On-line journal]. Available online at http://www.aafp.org/afp/20030215/tips/22.html.
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 http://www.aafp.org/afp/20030915/tips/6.html.
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 http://www.aafp.org/afp/20020215/tips/12.html.
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 http://www.aafp.org/afp/990201ap/635.html.
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 http://www.nlm.nih.gov/medlineplus/ency/article/003652.htm. Accessed August 2008.
(Updated May 29, 2007) Prothrombin Time. Massachusetts General Hospital, Pathology Service. Available online at http://www.massgeneral.org/pathology/coagbook/co004400.htm. 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 http://www.ncbi.nlm.nih.gov. Accessed August 2008.
Heart Rhythm Society. Patient Information, International Normalized Ratio. PDF available for download at http://www.heartrhythmfoundation.org/a-fib/INR_FINAL.pdf. Accessed August 2008.
University of Alabama at Birmingham Coagulation Service. Bleeding Guidelines. Available online at http://coag.path.uab.edu/bleeding.htm. Accessed August 2008.
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 http://www.medscape.com/viewarticle/758067. 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 http://www.medscape.com/viewarticle/759433. 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 http://www.medscape.com/viewarticle/737622. Accessed May 2012.
Dugdale, D. (Updated 2011 February 13). Prothrombin time (PT). MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003652.htm. Accessed May 2012.
(2014 November 18, Updated). A Patient’s Guide to Taking Warfarin. American Heart Association. Available online at http://www.heart.org/HEARTORG/Conditions/Arrhythmia/PreventionTreatmentofArrhythmia/A-Patients-Guide-to-Taking-Warfarin_UCM_444996_Article.jsp. Accessed on 4/05/15.
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