At a Glance
Why Get Tested?
To detect antibodies against the anticoagulant heparin, to help diagnose immune-mediated heparin-induced thrombocytopenia (HIT II)
When to Get Tested?
A blood sample drawn from a vein in your arm
Test Preparation Needed?
The Test Sample
What is being tested?
This test detects and measures antibodies that are produced by some people when they are treated with heparin. Heparin is a common anticoagulant that is given intravenously or through injections to prevent the formation of inappropriate blood clots (thrombosis) or as an initial treatment for those who have a blood clot, to prevent the clot from enlarging. It is often given during some surgeries, such as cardiopulmonary bypass, when the risk for developing blood clots is high. Small amounts of heparin are frequently used to flush out catheters and intravenous lines to keep clots from forming in them.
When a person is given heparin, the drug can combine with a substance found in platelets called platelet factor 4 (PF4) and form a complex. In some people, the body's immune system recognizes the heparin-PF4 complex as "foreign" and produces an antibody directed against it. This antibody can activate platelets and lead to a drop in the number of platelets, a condition known as heparin-induced thrombocytopenia (HIT). It may also lead to the development of new thrombosis or worsening thrombosis.
Platelets are cell fragments that are an important part of the blood clotting system. When a blood vessel is injured and leaks blood, platelets are activated and clump together at the site of the injury, and work with coagulation factors to promote clot formation and stop the bleeding.
Not everyone on heparin produces HIT antibodies, and not everyone with HIT antibodies develops a low platelet count, but about 1% to 5% of those with the antibodies do. In HIT, the antibodies bind to the heparin-PF4 complexes, which then attach to the surface of platelets. This activates the platelets, which in turn, triggers the release of more PF4. This starts a cycle that can cause a rapid and significant drop (e.g., 50% or more) in the number of platelets in the blood. Usually, a decrease in platelets results in a higher risk of bleeding, but in HIT, the activation of platelets by HIT antibodies can paradoxically lead to new and progressive blood clot formation in the veins and arteries. This occurs in about 30% to 50% of those who have the HIT antibody and thrombocytopenia.
This condition, associated with the presence of HIT antibody, low platelet count, and excessive clotting, is formally called immune-mediated heparin-induced thrombocytopenia or HIT type II. It typically develops about 5-10 days after a person starts heparin therapy but may also develop rapidly, within 1-2 days, if a person has been treated with heparin in the last 3 months and starts treatment again.
There is also a non-immune mediated HIT (type I) that occurs when heparin binds directly to platelets, causing activation; it is more common than type II but is transient and a milder form.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
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.
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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.
(© 1995-2011). Unit Code 81904: Heparin-PF4 Antibody (HIT), Serum. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/81904 through http://www.mayomedicallaboratories.com. Accessed September 2011.
Rodgers, III, G. and Smock, K. (Updated 2011 July). Heparin-Associated Antibody Syndrome – HIT. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/HIT.html?client_ID=LTD through http://www.arupconsult.com. Accessed September 2011.
Lefkowitz, J. (Updated 2010 July 20). Heparin-InducedThrombocytopenia. CAP from An Algorithmic Approach to Hemostasis Testing (2008) [On-line information]. Available online through http://www.cap.org. Accessed September 2011.
(© 2008). A Patient's Guide to Antithrombotic and Thrombolytic Therapy. American College of Chest Physicians [On-line information]. Available online at http://accpstorage.org/newOrganization/patients/AT8/AT8ComprehensiveGuidePatient.pdf through http://accpstorage.org. Accessed September 2011.
George, J. (Revised 2009 May). Thrombocytopenia: Other Causes. Merck Manual for Healthcare Professionals [On-line information]. Available online at http://www.merckmanuals.com/professional/sec12/ch143/ch143f.html?qt=heparin induced&alt=sh#v971368 through http://www.merckmanuals.com. Accessed September 2011.
Dugdale, D. (Updated 2010 February 5). Thrombocytopenia. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000586.htm. Accessed September 2011.
Dugdale, D. (Updated 2011 June 13). Thrombocytopenia - drug induced. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000556.htm. Accessed September 2011.
(2010 August 1). What Causes Thrombocytopenia? National Heart Lung and Blood Institute [On-line information]. Available online at http://www.nhlbi.nih.gov/health/health-topics/topics/thcp/causes.html through http://www.nhlbi.nih.gov. Accessed September 2011.
Baroletti S. and Samuel Z. Goldhaber, S. (2006 August 22). Heparin-Induced Thrombocytopenia. American Heart Association, from Circulation 2006, 114:e355-e356 [On-line information]. PDF available for download at http://circ.ahajournals.org/content/114/8/e355.full.pdf through http://circ.ahajournals.org. Accessed September 2011.
Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 753-755.
Wu, A. (© 2006). Tietz Clinical Guide to Laboratory Tests, 4th Edition: Saunders Elsevier, St. Louis, MO. Pp 534-535.
Henry's Clinical Diagnosis and Management by Laboratory Methods. 21st ed. McPherson R, Pincus M, eds. Philadelphia, PA: Saunders Elsevier: 2007 Pp 756, 781.
(Sep 23, 2010) Eke, S. Heparin-Induced Thrombocytopenia. Medscape Reference. Available online at http://emedicine.medscape.com/article/1357846-overview#a0199 through http://emedicine.medscape.com. Accessed Sept. 2011.