Also Known As
Heparin-PF4 Antibody
HIT Antibody
HIT PF4 Antibody
Heparin Induced Antibody
Heparin Associated Antibody
Formal Name
Heparin-induced Thrombocytopenia Platelet Factor 4 Antibody
This article was last reviewed on
This article waslast modified on March 11, 2020.
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?

When you are receiving heparin therapy and your platelet count significantly decreases (thrombocytopenia), especially when you also have new blood clots (thrombosis)

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?


You may be able to find your test results on your laboratory's website or patient portal. However, you are currently at Lab Tests Online. You may have been directed here by your lab's website in order to provide you with background information about the test(s) you had performed. You will need to return to your lab's website or portal, or contact your healthcare practitioner in order to obtain your test results.

Lab Tests Online is an award-winning patient education website offering information on laboratory tests. The content on the site, which has been reviewed by laboratory scientists and other medical professionals, provides general explanations of what results might mean for each test listed on the site, such as what a high or low value might suggest to your healthcare practitioner about your health or medical condition.

The reference ranges for your tests can be found on your laboratory report. They are typically found to the right of your results.

If you do not have your lab report, consult your healthcare provider or the laboratory that performed the test(s) to obtain the reference range.

Laboratory test results are not meaningful by themselves. Their meaning comes from comparison to reference ranges. Reference ranges are the values expected for a healthy person. They are sometimes called "normal" values. By comparing your test results with reference values, you and your healthcare provider can see if any of your test results fall outside the range of expected values. Values that are outside expected ranges can provide clues to help identify possible conditions or diseases.

While accuracy of laboratory testing has significantly evolved over the past few decades, some lab-to-lab variability can occur due to differences in testing equipment, chemical reagents, and techniques. This is a reason why so few reference ranges are provided on this site. It is important to know that you must use the range supplied by the laboratory that performed your test to evaluate whether your results are "within normal limits."

For more information, please read the article Reference Ranges and What They Mean.

What is being tested?

Heparin-induced thrombocytopenia is a complication of treatment with the blood-thinner (anticoagulant) heparin that can cause low platelets in the blood and an increased risk of blood clotting. This test detects and measures antibodies that are produced by some people when or after 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.

Sometimes, 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.

Accordion Title
Common Questions
  • How is it used?

    A test for heparin-induced thrombocytopenia (HIT) antibody, also called heparin-PF4 antibody, is performed to detect antibodies that develop in some people who have been treated with heparin. It is used to help establish a diagnosis of immune-mediated heparin-induced thrombocytopenia (HIT type II) in someone who has a low platelet count (thrombocytopenia) and excessive clotting (thrombosis).

    Heparin is an anticoagulant used to treat blood clots. In the body, heparin can combine with a substance call platelet factor 4 (PF4) to form a complex. Some people treated with heparin produce antibodies directed against this complex (HIT antibodies). For more on this, see the "What is being tested?" section.

    A person who has HIT antibodies will not necessarily develop HIT II. Therefore, this test is most useful in those with a moderate to high likelihood of having HIT II, based upon the timing of heparin use, significant thrombocytopenia, and thrombosis. The test is typically ordered along with or following a platelet count and may be followed by additional tests such as functional assays to confirm a finding.

    Functional assays, such as a serotonin release assay or heparin-induced platelet activation assay, are more specific for HIT II but take longer, are more technically demanding, and not widely available. These tests measure the effect a person's serum has on the function of "normal" platelets from healthy donors.

  • When is it ordered?

    Since the development of HIT antibodies does not always lead to HIT II, testing is usually ordered only when HIT II is clinically suspected.

    There is a pre-test scoring system that is typically used to determine a person's likelihood of having HIT II. It includes:

    • The extent of thrombocytopenia (platelet decrease of 50% or more from the pre-heparin therapy level)
    • The timing of the platelet count fall (typically 5-10 days after initial heparin use and within 2 day for a second use within 3 months of previous use)
    • The presence of new blood clots (thrombosis) and/or lesions at the heparin injection site
    • Ruling out other causes of low platelet count

    The HIT antibody test is performed when this pre-scoring test shows that a person has a moderate to high likelihood of having HIT II.

    Typically, an enzyme immunoassay (EIA) that detects HIT antibody is ordered as an initial test. Functional testing such as a serotonin release assay (SRA) or heparin-induced platelet activation (HIPA) test may be ordered when the EIA test is indeterminate or negative but suspicion of HIT is still high.

  • What does the test result mean?

    The interpretation of HIT antibody results relies upon testing only people who have a moderate to high probability of having HIT II. Both false negatives and false positives can occur with this test and are more likely in those with a low probability of having HIT II.

    The presence of HIT antibodies in someone who has been treated with heparin for 5 to 10 days, has a platelet count that has decreased by 50% or more, and has new blood clots means that it is likely the person has HIT II.

    The presence of HIT antibodies in someone who has received heparin within the last 3 months and is experiencing significant thrombocytopenia within a day or two of re-starting heparin therapy may also indicate HIT II.

    If HIT testing is indeterminate and confirmatory testing is positive in a person with clinical signs of HIT, then it is likely the person has HIT II.

    If the test is negative for HIT antibodies,then it is unlikely that the person has HIT II. If confirmatory testing is performed and it is also negative, then it is likely that the person's symptoms are due to another cause.

  • Is there anything else I should know?

    The majority of people who produce HIT antibodies will not develop HIT II (i.e., have significant thrombocytopenia and thrombosis).

    Many conditions and diseases other than HIT can cause thrombocytopenia by affecting platelet production or increasing platelet loss (destruction). In addition to heparin, there are several other medications that can cause drug-induced thrombocytopenia and antiplatelet antibodies.

    Heparin-induced thrombocytopenia type I (HIT type I) may be seen in people who are receiving heparin, but HIT I tends to be a more mild condition that is not associated with an immune reaction and is typically of no clinical significance.

    There are two types of heparin that may be used in treatment: standard or unfractionated heparin (UFH) and low-molecular weight heparin (LMWH). HIT II can develop in anyone receiving UFH but is more likely in those who have had surgery. The condition is rare in children. Low molecular weight heparin (LMWH) does not generally cause HIT II, but it can. Once a person has developed HIT II with UFH, they are more likely to develop HIT with LMWH.

    It is rare but possible for people to develop HIT antibodies, even when the only heparin that they are exposed to is the small amount used to flush out their intravenous line or catheter

  • Can the heparin-induced thrombocytopenia (HIT) antibody test be done in my doctor’s office?

    No. It requires specialized equipment and is not offered by every hospital-based laboratory. It may be necessary to send your blood to a reference laboratory.

  • If I have an HIT antibody, will it go away?

    The amount of antibody will generally decrease after several months, but you may develop it again if you are given more heparin.

  • How long is someone usually treated with heparin?

    In most cases, a person is on heparin for a short period of time and then transitioned to another anticoagulant (e.g., oral Coumadin). Pregnant women who need anticoagulation may receive low molecular weight heparin (LMWH) for extended periods of time.

  • Should I tell all of my doctors that I have an HIT antibody?

    Yes. This is important information for your healthcare practitioners to know as it may affect other procedures and some of your treatment options.

View Sources

Sources Used in Current Review

(October 17, 2015) Eke S. Heparin-Induced Thrombocytopenia. Medscape Reference. Available online at Accessed February 2016.

Wintrobe's Clinical Hematology. 12th ed. Greer J, Foerster J, Rodgers G, Paraskevas F, Glader B, Arber D, Means R, eds. Philadelphia, PA: Lippincott Williams & Wilkins: 2009, Pp 1497-1499.

Lee G, Arepally G. Diagnosis and Management of Heparin-Induced Thrombocytopenia. Hematol Oncol Clin North Am. 2013 Jun; 27(3): 541–563. Available online at Accessed February 2016.

Sources Used in Previous Reviews

(© 1995-2011). Unit Code 81904: Heparin-PF4 Antibody (HIT), Serum. Mayo Clinic Mayo Medical Laboratories [On-line information]. Available online at 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 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 Accessed September 2011.

(© 2008). A Patient's Guide to Antithrombotic and Thrombolytic Therapy. American College of Chest Physicians [On-line information]. Available online at Accessed September 2011.

George, J. (Revised 2009 May). Thrombocytopenia: Other Causes. Merck Manual for Healthcare Professionals [On-line information]. Available online at induced&alt=sh#v971368. Accessed September 2011.

Dugdale, D. (Updated 2010 February 5). Thrombocytopenia. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed September 2011.

Dugdale, D. (Updated 2011 June 13). Thrombocytopenia - drug induced. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed September 2011.

(2010 August 1). What Causes Thrombocytopenia? National Heart Lung and Blood Institute [On-line information]. Available online at 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 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 Accessed Sept. 2011.

Ask a Laboratory Scientist

This form enables patients to ask specific questions about lab tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. Please allow 2-3 business days for an email response from one of the volunteers on the Consumer Information Response Team.

Thank you for using the Consumer Information Response Service ("the Service") to inquire about the meaning of your lab test results.  The Service is provided free of charge by the American Society for Clinical Laboratory Science, which is one of many laboratory organizations that supports Lab Tests Online.
Please note that information provided through this free Service is not intended to be medical advice and should not be relied on as such. Although the laboratory provides the largest single source of objective, scientific data on patient status, it is only one part of a complex biological picture of health or disease. As professional clinical laboratory scientists, our goal is to assist you in understanding the purpose of laboratory tests and the general meaning of your laboratory results. It is important that you communicate with your physician so that together you can integrate the pertinent information, such as age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), to determine your health status. The information provided through this Service is not intended to substitute for such consultations with your physician nor specific medical advice to your health condition.
By submitting your question to this Service, you agree to waive, release, and hold harmless the American Society for Clinical Laboratory Science and its affiliates or their past or present officers, directors, employees, agents, and Service volunteers (collectively referred to as "ASCLS") and the American Association  for Clinical Chemistry and its affiliates or their past or present officers, directors, employees, agents, and Service volunteers (collectively referred to as "AACC") from any legal claims, rights, or causes of action you may have in connection with the responses provided to the questions that you submit to the Service.
AACC, ASCLS and its Service volunteers disclaim any liability arising out of your use of this Service or for any adverse outcome from your use of the information provided by this Service for any reason, including but not limited to any misunderstanding or misinterpretation of the information provided through this Service.