Partial Thromboplastin Time (PTT, aPTT)
- Also Known As:
- PTT Activated Partial Thromboplastin Time APTT aPTT
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At a Glance
Why Get Tested?
As part of an investigation of a possible bleeding disorder or blood clot (thrombotic episode); to help investigate recurrent miscarriages or diagnose antiphospholipid syndrome (APS); as needed to monitor unfractionated (standard) heparin anticoagulant therapy; as indicated as part of an evaluation before surgery or other invasive procedure
When To Get Tested?
When you have unexplained bleeding, inappropriate blood clotting, or recurrent miscarriages; sometimes when you are on standard heparin anticoagulant therapy; sometimes before a scheduled surgery
A blood sample drawn by needle from a vein in your arm
Test Preparation Needed?
None; however, a high-fat meal prior to the blood draw may interfere with the test and should be avoided.
What is being tested?
The partial thromboplastin time (PTT; also known as activated partial thromboplastin time (aPTT)) is a screening test that helps evaluate a person’s ability to appropriately form blood clots. It measures the number of seconds it takes for a clot to form in a sample of blood after substances (reagents) are added. The PTT assesses the amount and the function of certain proteins in the blood called coagulation or clotting factors that are an important part of blood clot formation.
When body tissue(s) or blood vessel walls are injured, bleeding occurs and a process called hemostasis begins. Small cell fragments called platelets stick to and then clump (aggregate) at the injury site. At the same time, a process called the coagulation cascade begins and coagulation factors are activated in a step-by-step process. Through the cascading reactions, threads called fibrin form and crosslink into a net that clings to the injury site and stabilizes it. This forms a stable blood clot to seal off injuries to blood vessels, prevents additional blood loss, and gives the damaged areas time to heal.
Each part of this hemostatic process must function properly and be present in sufficient quantity for normal blood clot formation. If the amount of one or more factors is too low, or if the factors cannot do their job properly, then a stable clot may not form and bleeding continues.
With a PTT, your result is compared to a normal reference interval for clotting time. When your PTT takes longer than normal to clot, the PTT is considered “prolonged.”
When a PTT is used to investigate bleeding or clotting episodes or to rule out a bleeding or clotting disease (e.g., preoperative evaluation), it is often ordered along with a prothrombin time (PT). A health care practitioner will evaluate the results of both tests to help rule out or determine the cause of bleeding or clotting disorder.
It is now understood that coagulation tests such as the PT and PTT are based on what happens artificially in the test setting (in vitro) and thus do not necessarily reflect what actually happens in the body (in vivo). Nevertheless, they can be used to evaluate certain components of the hemostasis system. The PTT and PT tests each evaluate coagulation factors that are part of different groups of chemical reaction pathways in the cascade, called the intrinsic, extrinsic, and common pathways.
- The PTT is used to evaluate the coagulation factors XII, XI, IX, VIII, X, V, II (prothrombin), and I (fibrinogen) as well as prekallikrein (PK) and high molecular weight kininogen (HK).
- A PT test evaluates the coagulation factors VII, X, V, II, and I (fibrinogen).
How is it used?
The PTT is used primarily to investigate unexplained bleeding or clotting. It may be ordered along with a prothrombin time (PT/INR) to evaluate the process that the body uses to form blood clots to help stop bleeding. These tests are usually the starting points for investigating excessive bleeding or clotting disorders.
By evaluating the results of the two tests together, a healthcare practitioner can gain clues as to what bleeding or clotting disorder may be present. The PTT and PT are not diagnostic but usually provide information on whether further tests may be needed.
Some examples of uses of a PTT include:
- To identify coagulation factor deficiency; if the PTT is prolonged, further studies can then be performed to identify what coagulation factors may be deficient or dysfunctional, or to determine if an antibody against a coagulation factor (known as a factor-specific inhibitor) is present in the blood.
- To detect nonspecific autoantibodies (antiphospholipid antibodies), such as lupus anticoagulant; these are associated with clotting episodes and with recurrent miscarriages. For this reason, PTT testing may be performed as part of a clotting disorder panel to help investigate recurrent miscarriages or diagnose antiphospholipid syndrome (APS). A variation of the PTT called the LA-sensitive PTT may be used for this purpose.
- To monitor standard (unfractionated, UF) heparin anticoagulant therapy; however, some labs now use the anti-Xa test to monitor heparin therapy. Heparin is an anticoagulation drug that is given intravenously (IV) or by injection to prevent and to treat blood clots (embolism and thromboembolism). It prolongs PTT. When heparin is administered for therapeutic purposes, it must be closely monitored. If too much is given, the treated person may bleed excessively; with too little, the treated person may continue to clot.
- Based on carefully obtained patient histories, the PTT and PT are sometimes selectively performed before a scheduled surgery or other invasive procedures to screen for potential bleeding tendencies.
When is it ordered?
The PTT may be ordered along with other tests such as a PT when you have:
- Unexplained bleeding or easy bruising
- A blood clot in a vein or artery
- An acute condition such as disseminated intravascular coagulation (DIC) that may cause both bleeding and clotting as coagulation factors are used up at a rapid rate
- A chronic condition such as liver disease that may affect clotting
A PTT may be ordered:
- As part of an evaluation for lupus anticoagulant, anticardiolipin antibodies, and antiphospholipid syndrome, when you’ve had a blood clot or when a woman has had recurrent miscarriages
- When you are switched from heparin therapy to longer-term warfarin (Coumadin®) therapy, the two are overlapped and both the PTT and PT are monitored until you have stabilized.
- When you have a surgical operation scheduled; you may have a PTT prior to surgery when the surgery carries an increased risk of blood loss and/or when you have a clinical history of bleeding, such as frequent or excessive nose bleeds and easy bruising, which may indicate the presence of a bleeding disorder.
What does the test result mean?
PTT results are typically reported in seconds.
A PTT result that falls within a laboratory’s reference interval usually indicates normal clotting function. However, even with a normal PTT result, mild to moderate deficiencies of a single coagulation factor may be present. The PTT may not be prolonged until the factor levels have decreased to 30% to 40% of normal. On the other hand, lupus anticoagulant may be present but may not prolong the PTT result. If the lupus anticoagulant (LA) is suspected, a more sensitive LA-sensitive PTT or a dilute Russell viper venom time (DRVVT) can be used to test for it. (See below for more about LA-sensitive PTT.)
A prolonged PTT means that clotting is taking longer to occur than normal and may be due to a variety of causes.
A prolonged PTT may be due to:
- Underlying conditions that cause low levels of clotting factors, such as:
- Liver disease—most coagulation factors are produced by the liver, thus liver disease may cause prolonged PT and PTT. However, PT is more likely to be prolonged than PTT.
- Vitamin K deficiency—vitamin K is essential for the formation of several clotting factors. Vitamin K deficiencies are rare but can be caused by an extremely poor diet, malabsorption disorders, or prolonged use of certain antibiotics, for example. PT is more likely to be prolonged than is PTT.
- Less common inherited clotting factor deficiencies:
- von Willebrand disease (vWD) is the most common inherited bleeding disorder and it affects platelet function due to decreased von Willebrand factor. PTT is normal in most cases of vWD but can be prolonged in severe vWD.
- Hemophilia A and hemophilia B (Christmas disease) are two other inherited bleeding disorders resulting from a decrease in factors VIII and IX, respectively.
- Deficiencies of other coagulation factors, like factors XII and XI. Deficiency in XI can cause abnormal bleeding, but deficiency of XII is not associated with bleeding risk in the body.
- A nonspecific inhibitor such as the lupus anticoagulant— this is an autoantibody (antiphospholipid antibody) that interferes with the PTT because it targets substances called phospholipids that are used in the PTT. Though they can prolong the PTT result, in the body they are associated with excessive clotting. A person who produces these antibodies may be at an increased risk for a blood clot.
- A specific inhibitor—although relatively rare, these are antibodies that specifically target certain coagulation factors (known as factor-specific inhibitor), affecting how they function. An example is an antibody that targets factor VIII. Factor-specific inhibitors can cause severe bleeding.
- Heparin—is an anticoagulant and will prolong a PTT, either as a contaminant of the sample or as part of anticoagulation therapy. For anticoagulant therapy, the target PTT is often about 1.5 to 2.5 times longer than a person’s pretreatment level. PTT is still being used to monitor standard heparin therapy.
- Warfarin (Coumadin®) anticoagulation therapy—the PTT is not used to monitor warfarin therapy, but PTT may be prolonged by warfarin at high dose. Typically, the prothrombin time/international normalized ratio (PT/INR) is used to monitor warfarin therapy.
- Other anticoagulants—anticoagulation therapy with direct thrombin inhibitor (e.g., argatroban, dabigatran) or direct factor Xa inhibitor (e.g., rivaroxaban, apixaban)
- Prolonged PTT levels may also be seen with certain types of leukemia, excessive bleeding in pregnant women prior to or after giving birth, or recurrent miscarriages.
Results of the PTT are often interpreted with results of the PT in determining what condition may be present.
|PT result||ptt result||Common condition present|
|Prolonged||Normal||Liver disease, vitamin K deficiency, decreased or defective factor VII, chronic DIC, warfarin or other vitamin K antagonist (e.g., brodifacoum in some cannabinoids)|
|Normal||Prolonged||Hemophilia A or B (decreased or defective factor VIII or IX) or factor XI deficiency, von Willebrand disease (severe form), factor XII deficiency, or lupus anticoagulant present|
|Prolonged||Prolonged||Decreased or defective factor I (fibrinogen), II (prothrombin), V or X, severe liver disease, acute DIC|
|Normal||Normal or slightly prolonged||May indicate normal hemostasis; however, PT and PTT can be normal in conditions such as mild deficiencies in other factors and mild form of von Willebrand disease. Further testing may be required to diagnose these conditions.|
A shortened PTT may be due to:
- Disseminated intravascular coagulation (DIC)—in the early stages of DIC, there are circulating procoagulants that shorten the PTT.
- Advanced cancer (ovarian, pancreatic, colon), except when the liver is involved
- An acute-phase reaction: this is a condition causing extensive tissue inflammation or trauma that significantly elevates factor VIII levels. It is usually a temporary change that is not monitored with a PTT. When the condition causing the acute phase reaction is resolved, the PTT returns to normal.
What is an LA-sensitive PTT and how does it differ from regular PTT?
The LA-sensitive PTT (LA-PTT or PTT-LA) is a variant PTT, designed to evaluate the presence of lupus anticoagulant (LA), an antibody associated with clotting episodes and recurrent miscarriages. The LA-PTT uses a low phospholipid reagent that is optimized for detecting lupus anticoagulants and is therefore more sensitive to LA. The test is based on the principle that lupus anticoagulant binds to the phospholipids that are used as one of the reagents in the PTT test, causing an abnormally prolonged clotting time.
In addition to a PTT, what other tests might be done?
Examples of other testing that may be done along with a PTT or in follow up to abnormal results include:
- Platelet count – should always be monitored during heparin therapy to promptly detect any heparin-induced thrombocytopenia
- Thrombin time testing – sometimes ordered to help rule out heparin contamination
- Fibrinogen testing – may be done to rule out a low level of fibrinogen as a cause of a prolonged PTT
- When an initial PTT is prolonged, a second PTT test is performed by mixing the person’s plasma with pooled normal plasma (a collection of plasma from a number of normal donors). If the PTT time returns to normal (“corrects”), it suggests a deficiency of one or more of the coagulation factors in the person’s plasma. If the time remains prolonged, then the problem may be due to the presence of an abnormal factor-specific factor inhibitor (autoantibody) or nonspecific inhibitor, such as lupus anticoagulant.
- Coagulation factor tests – these measure the activity (function) of coagulation factors. They can detect reduced levels of the protein or proteins that don’t work properly (have reduced function). Rarely, the antigen level (quantity) of a coagulation factor may also be measured.
- Dilute Russell viper venom test (DRVVT) – a test that may be done if the presence of lupus anticoagulant is suspected.
- von Willebrand factor – sometimes ordered to help determine if von Willebrand disease (severe form) is the cause of a prolonged PTT.
Is the PTT always used to monitor heparin therapy?
In a few situations, it is not.
- When very high doses of heparin are used, as may occur during open-heart surgery, the PTT loses its sensitivity; it will not clot. At this intense level of anticoagulation, the activated clotting time (ACT) can be used as a monitoring tool.
- Some hospitals now monitor standard (unfractionated) heparin therapy using the chromogenic anti-factor Xa test in lieu of PTT.
- Low molecular weight heparin (LMWH) is a fast-acting form of heparin often used in the treatment of conditions such as deep vein thrombosis (DVT) prevention. Though generally not requiring monitoring, it must be monitored using the anti-factor Xa test because LMWH typically does not prolong PTT.
- For people with lupus anticoagulant and clotting and who are being treated with heparin, the PTT is not reliable; thus the anti-factor Xa test must be used to monitor their heparin therapy.
How can I change my PTT?
The PTT is not something you can change through lifestyle changes (unless you have a vitamin K deficiency). It is a reflection of the integrity of your clotting system. If your PTT is prolonged due to acquired factor deficiencies, then addressing the underlying condition may bring the results to near normal levels. If they are prolonged due to a temporary or acute condition, they should return to normal on their own when the acute condition is resolved. Inherited coagulation abnormalities or deficiencies should be closely monitored and may be treated with frequent replacement infusions of the missing clotting factor.
Is there anything else I should know?
Two anticoagulants often used, low molecular weight heparin (LMWH) and danaparoid, may not prolong the PTT and, if indicated, should be monitored using the heparin anti-factor Xa assay.
Several factors can affect results of a PTT and the interpretation of test results:
- People with high hematocrit levels may have falsely prolonged PTTs.
- Heparin contamination – this is the most common problem, especially when blood is collected from intravenous lines that are being kept “open” with heparin flushes.
- Drugs such as antihistamines, vitamin C (ascorbic acid), aspirin, and chlorpromazine
- In some cases, heparin can unintentionally decrease a person’s platelet count in a complication called heparin-induced thrombocytopenia. When this occurs, substitute anticoagulants such as a direct thrombin inhibitor (e.g., argatroban or bivalirudin) may be given. The PTT test may also be used to monitor these therapies. It does not directly measure the anticoagulants used but measures their effect on blood clotting.
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|
|16631-4||aPTT Coagulation 1:1 saline (Bld) [Time]|
|3173-2||aPTT Coag (Bld) [Time]|
|49058-1||aPTT Coag (BldCRRT) [Time]|
|43734-3||aPTT Coagulation 1:1 saline (PPP) [Time]|
|14979-9||aPTT Coag (PPP) [Time]|
Sources Used in Current Review
Activated Partial Thromboplastin Time (APTT), Plasma. Mayo Medical Laboratories. Available online at https://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/40935. Accessed March 2019.
(November 23, 2015) Partial Thromboplastin Time, Activated. Medscape. Available online at https://emedicine.medscape.com/article/2085837-overview. Accessed March 2019.
(June 2013) Elevated APTT? How to Best Follow Up. Journal of Family Practice. Available online at https://www.mdedge.com/jfponline/article/76289/cardiology/elevated-aptt-how-best-follow. Accessed March 2019.
Partial Thromboplastin Time (PTT). MedlinePlus. Available online at https://medlineplus.gov/ency/article/003653.htm. Accessed March 2019.
Activated Partial Thromboplastin Time (APTT). University of Rochester Medical Center. Available online at https://www.urmc.rochester.edu/encyclopedia/content.aspx?contenttypeid=167&contentid=aptt. Accessed March 2019.
Vandiver JW, Vondracek T. Antifactor Xa levels versus activated partial thromboplastin time for monitoring unfractionated heparin. Pharmacotherapy. 2012 Jun;32(6):546-58. Available online at https://www.ncbi.nlm.nih.gov/pubmed/22531940. Accessed March 2019.
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.
(2002 November 19, Modified). Activated Partial Thromboplastin Time. Mass Gen. Hospital Pathology Service Laboratory Medicine [On-line information, Coag Test Handbook]. Available online at http://www.mgh.harvard.edu/labmed/lab/coag/handbook/co003400.htm#co003400.
Activated Partial Thromboplastin Time. MCL Web Resources, Topics in Hemostasis [On-line information]. Available online at http://22.214.171.124/hemostasis/PTT1.html.
Olson, J. (1999 September). Addressing clinical etiologies of a prolonged aPTT. CAP Today, In the News [On-line Newsletter]. Available online at http://www.cap.org/captoday/CaseStudy/coag4.html.
Activated Partial Thromboplastin Time. Florida Hospital Cancer Institute, Clinical and Research Laboratories Coagulation Test Panels [On-line information]. Available online at http://www.fhci-labs.com/researchlabs/clinicallabs/hemostasisandthrombosis/panels.htm.
PTT-LA. Florida Hospital Cancer Institute, Clinical and Research Laboratories Coagulation Test Panels [On-line information]. Available online at http://www.fhci-labs.com/researchlabs/clinicallabs/hemostasisandthrombosis/panels.htm.
Duke University Medical Center Clinical Coagulation Laboratory Coagulation Test Descriptions [On-line information]. Available online at http://pathology.mc.duke.edu/coag/TestDes.htm.
Elstrom, R. (2001 November 25, Updated ). PTT. MedlinePlus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003653.htm.
Coagulation Test Panels. Clinical and Research Laboratories, Florida Hospital Cancer Institute [On-line information]. Available online at http://www.fhci-labs.com/researchlabs/clinicallabs/hemostasisandthrombosis/panels.htm.
Menta, S. (1999 Spring). The Coagulation Cascade. Physiology Disorders Evaluation, College of Medicine, Univ of Florida [On-line information]. Available online at http://www.medinfo.ufl.edu/year2/coag/title.html.
Elstrom, R. (2001 November 25, Updated). PT. MedlinePlus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003652.htm.
Elstrom, R. (2001 November 25, Updated). PTT. MedlinePlus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003653.htm.
What is a Bleeding Disorder? National Hemophilia Foundation, Bleeding Disorders Info Center [On-line information]. Available online at http://www.hemophilia.org/bdi/bdi_general.htm.
DeLoughery, T. (1999 March 15). Tests of Hemostasis and Thrombosis. OHSU [Online student handout]. Available online at http://www.ohsu.edu/som-hemonc/handouts/deloughery/printtest.html.
Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosby’s Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 705-707.
Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. Harris, N. et. al. Chapter 19: Assessment of Hemostasis in the Clinical Laboratory. Pp 227-239.
Dugdale, D. (Updated 2010 August 19). Partial thromboplastin time (PTT). MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003653.htm. Accessed September 2010.
(© 1995–2010). Unit Code 9058: Activated Partial Thromboplastin Time (APTT), Plasma. Mayo Clinic, Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Overview/9058. Accessed September 2010.
Ballas, M. and Kraut, E. (2008 April 15). Bleeding and Bruising: A Diagnostic Work-up. Am Fam Physician. 2008 Apr 15;77(8):1117-1124. [On-line information]. Available online at http://www.aafp.org/afp/2008/0415/p1117.html. Accessed September 2010.
(© 1996 – 2010). 3895: Activated Partial Thromboplastin Time (APTT). Specialty Laboratories [On-line information]. Available online at http://www.specialtylabs.com/tests/details.asp?id=3895. September June 2010.
(© 1995-2010). Blood Test: Partial Thromboplastin Time (PTT). KidsHealth from Nemours [On-line information]. Available online at http://kidshealth.org/parent/system/medical/test_ptt.html. Accessed September 2010.
Moake, J. (Revised 2009 June). Uncommon Hereditary Coagulation Disorders. Merck Manual for Healthcare Professionals [On-line information]. Available online at http://www.merck.com/mmpe/sec11/ch136/ch136e.html. Accessed September 2010.
Pagana, K. D. & Pagana, T. J. (© 2007). Mosby’s Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 705-707.
Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 46-47, 932-933.
(January 2008) How to Interpret and Pursue an Abnormal Prothrombin Time, Activated Partial Thromboplastin Time and Bleeding Time. MyoMedialLaboratories.com, Vol. 33 No.1. PDF available for download at http://www.mayomedicallaboratories.com/mediax/articles/communique/2008/mc2831-0108.pdf. Accessed September 2010.
Jacobs DS, DeMott WR, Oxley DK. Laboratory Test Handbook, 5th ed.Lexicomp: Hudson, OH. Coagulation: EM Van Cott & M Laposata, Pp 327-358, 2001.
Florida Hospital, Center for Thrombosis Research, © 2008, PTT-LA. Available online at http://www.fhthrombosis.com/PTT-LA. Accessed October 2010.
Dugdale, D.C. (Updated 2011 February 13) Partial Thromboplastin Time (PTT). National Institutes of Health MedlinePlus. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003653.htm. Accessed May 2014.
Hammami, M. B. (Updated 2013 February 20). Partial Thromboplastin Time, Activated. Medscape. Available online at http://emedicine.medscape.com/article/2085837-overview. Accessed May 2014.
(© 1995–2014). Activated Partial Thromboplastin Time (APTT), Plasma. Mayo Medical Laboratories. Available online at http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/9058. Accessed May 2014.
Tuazon, S.A. et al. (Updated 2012 September 4). Prothrombin Time. Available online at http://emedicine.medscape.com/article/2086058-overview. Accessed May 2014.
Pollak, E.S. (Updated 2012 April 6). von Willebrand Disease. Medscape. Available online at http://emedicine.medscape.com/article/206996-overview. Accessed May 2014.
Pagana, K. D. & Pagana, T. J. (© 2014). Mosby’s Manual of Diagnostic and Laboratory Tests Reference 5th Edition: Mosby, Inc., Saint Louis, MO. Pp 383-386.
(Reviewed 2013 February). Recurrent Miscarriage Evaluation/Coagulation Panel. Quest Diagnostics. Available online at http://www.questdiagnostics.com/testcenter/testguide.action?dc=TS_Recurrent_Miscarriage. Accessed May 2014.
Moake, J. (Reviewed 2012 July). Excessive Bleeding. The Merck Manual. Available online at http://www.merckmanuals.com/professional/hematology_and_oncology/hemostasis/excessive_bleeding.html?qt=partial%20thromboplastin&alt=sh. Accessed May 2014.
Moake, J. (Reviewed 2013 December). Coagulation Disorders Caused by Circulating Anticoagulants. The Merck Manual. Available online through http://www.merckmanuals.com. Accessed May 2014.
Eke, Sancar. (Reviewed 2012 March). Medscape. Heparin-Induced Thrombocytopenia Treatment & Management. Available online at http://emedicine.medscape.com/article/1357846-treatment. Accessed May 2014.
Szigeti, Reka. (Updated 2012 October). Anti-Xa Assay (Heparin Assay). Medscape. Available online at http://emedicine.medscape.com/article/2085000-overview#a30. Accessed May 2014.