From: Elizabeth M. Van Cott, M.D., and Michael Laposata, M.D., Ph.D., “Coagulation.” In: Jacobs DS et al, ed. The Laboratory Test Handbook, 5th Edition. Lexi-Comp, Cleveland, 2001; 327-358.
Synonyms HIT
Applies to Argatroban; Coumadin®; Danaparoid; Heparin; Hirudin; PF4; Platelet Factor 4; Serotonin Release Assays
Abstract Heparin-induced thrombocytopenia (HIT) is a common, serious complication of heparin therapy, with a high risk of potentially catastrophic venous or arterial thrombosis and high mortality.
Specimen Plasma (some laboratories may use serum)
Container Blue top (sodium citrate) tube for plasma (or red top tube if serum is requested); one tube suffices for ELISA or platelet aggregation, more tubes may be required for serotonin release assay.
Collection Routine venipuncture
Storage Instructions Plasma (or serum) can be stored for 24 hours at room temperature; otherwise, store frozen.
Turnaround Time 1 day, unless testing is batched less frequently
Special Instructions For serotonin release or platelet aggregation assays, notify laboratory if patient is receiving heparin. Such specimens should ideally not contain heparin. If heparin is present, it may be removed (adsorbed) by the laboratory prior to testing.
Reference Interval Negative for HIT antibody (HIT antibody not present)
Use Determine if thrombocytopenia or thrombosis in a patient exposed to heparin is due to heparin-induced thrombocytopenia
Limitations The antibody disappears after heparin is discontinued, usually within weeks to months but occasionally longer. Therefore, testing should be performed in the acute setting when HIT is presently suspected.
Methodology Three methods are commonly in use. Enzyme-linked immunosorbent assays (ELISA) use heparin complexed to platelet factor 4 (PF4) as the antigen. In platelet aggregation assays, patient plasma (or serum) is added to normal donor platelets and heparin. If the HIT antibody is present, it stimulates the platelets to aggregate. In serotonin release assays, patient plasma (or serum) and heparin are added to normal platelets that contain radiolabeled serotonin. If the HIT antibody is present, it activates the platelets which then release their serotonin. The released radiolabeled serotonin can then be detected.
Additional Information Thrombocytopenia and thrombosis are the predominant clinical features of HIT. Despite the thrombocytopenia, bleeding complications are uncommon.1 Up to 8% of heparinized patients develop the antibody that causes HIT without becoming thrombocytopenic.2,3 Another 1% to 5% of patients on heparin progress further to HIT with thrombocytopenia,3,4,5,6 and of those, at least 33% develop venous and/or arterial thrombosis.3,5,6,7,8,9 Thrombosis usually occurs only in HIT patients who are thrombocytopenic. However, thrombosis has been reported in HIT patients with normal platelet counts.10 Thrombosis in HIT is associated with a mortality of approximately 20% to 30%, with an equal number becoming permanently disabled by amputation, stroke, or other causes.1,7,8,11 HIT can develop from even small amounts of heparin, such as line flushes or heparin-coated catheters.
In patients receiving heparin for the first time, the platelet count begins to decrease in HIT 4-20 days after initiation of heparin exposure, most commonly between days 5 and 12, with the median on day 10.1,12 In patients who were sensitized to heparin in the past, platelet counts may decrease within the first 3 days or even hours after re-exposure to heparin.1,13 A progressive decline in platelet count >50% from baseline or to <100,000/microL is typical of HIT. The median nadir is 50,000/microL (range 20,000-150,000/microL). In patients developing HIT for the first time, the nadir is reached about 5 days after the onset of the decline, although this is variable. In previously sensitized patients, the nadir can be reached as soon as the first day or two after heparin re-exposure. After discontinuing heparin, the platelet count starts to rise after 2-3 days and usually returns to normal within 4-10 days. However, occasionally the recovery requires up to 25 days.1,13 HIT is due to an antibody that recognizes heparin bound to platelet factor 4 (PF4) on the platelet surface. The antibody binds to the heparin-PF4 complex, which then allows the antibody to bind the Fc receptor on the platelet.14 Interaction with the Fc receptor activates the platelet, resulting in platelet loss (thrombocytopenia) and platelet aggregation (thrombosis). A minority of cases of HIT may involve an antigen other than the PF4-heparin complex.15 Among the HIT tests, the ELISA is the most sensitive and platelet aggregation the least sensitive.16,17,18 The sensitivity of serotonin release or ELISA is greater than or equal to 90%. Thus, a negative test for HIT does not rule out the diagnosis with complete certainty if HIT is suspected clinically. All three tests have high specificity. However, the significance of a positive ELISA in the absence of thrombocytopenia or thrombosis is uncertain. There is at least one case reported in which the antibody was detected by ELISA 5 days prior to the onset of thrombocytopenia.18 Heparin should be permanently discontinued in HIT patients (special arrangements are made for patients who require bypass surgery). Platelet transfusions should be avoided. Patients with HIT are often treated with danaparoid, hirudin, or argatroban. Low-molecular weight heparin (LMWH) has a lower incidence of HIT than unfractionated heparin.3 However, the cross-reactivity of the HIT antibody against LMWH is high enough that LMWH is also contraindicated for HIT patients, now that the newer alternatives mentioned above are available. Coumadin® should not be used alone in the setting of acute HIT, because it may precipitate venous limb gangrene.19 If Coumadin® is used, an immediate-acting alternative anticoagulant (eg, hirudin, danaparoid, argatroban) should be used with it until Coumadin® is therapeutic. Footnotes
1. Greinacher A, “Antigen Generation in Heparin-Associated Thrombocytopenia: The Nonimmunologic Type and the Immunologic Type Are Closely Linked in Their Pathogenesis,”Semin Thromb Hemost, 1995, 21(1):106-16.
2. Kappers-Klunne MC, Boon DMS, Hop WC, et al, “Heparin-Induced Thrombocytopenia and Thrombosis: A Prospective Analysis of the Incidence in Patients With Heart and Cerebrovascular Diseases,”Br J Haematol, 1997, 96(3):442-6.
3. Warkentin TE, Levine MN, Hirsch J, et al, “Heparin-Induced Thrombocytopenia in Patients Treated With Low-Molecular Weight Heparin or Unfractionated Heparin,”N Engl J Med, 1995, 332(20):1330-5.
4. Schmitt BP and Adelman B, “Heparin-Associated Thrombocytopenia: A Critical Review and Pooled Analysis,”Am J Med Sci, 1993, 305(4):208-15.
5. Warkentin TE and Kelton JG, “Interaction of Heparin With Platelets, Including Heparin-Induced Thrombocytopenia,”Low-Molecular Weight Heparins in Prophylaxis and Therapy of Thromboembolic Diseases, Bounameaux H, ed, New York, NY: Marcel Dekker Inc, 1994, 75-127.
6. Baglin TP, “Heparin-Induced Thrombocytopenia/Thrombosis Syndrome (HIT): Diagnosis and Treatment,”Platelets, 1997, 8:72-4.
7. Demasi R, Bode AP, Knupp C, et al, “Heparin-Induced Thrombocytopenia,”Am J Surg, 1994, 60(1):26-9.
8. Nand S, Wong W, Yuen B, et al, “Heparin-Induced Thrombocytopenia With Thrombosis: Incidence, Analysis of Risk Factors, and Clinical Outcomes in 108 Consecutive Patients Treated at a Single Institution,”Am J Hematol, 1997, 56(1):12-6.
9. Warkentin TE and Kelton JG, “A 14-Year Study of Heparin-induced Thrombocytopenia,”Am J Med, 1996, 101(5):502-7.
10. Hach-Wunderle V, Kainer K, Salzmann G, et al, “Heparin-Related Thrombosis Despite Normal Platelet Counts in Vascular Surgery,”Am J Surg, 1997, 173(2):117-9.
11. Magnani HN, “Heparin-Induced Thrombocytopenia (HIT): An Overview of 230 Patients Treated With Orgaran (Org 10172),”Thromb Haemost, 1993, 70(4):554-61.
12. King DJ and Kelton JG, “Heparin-Associated Thrombocytopenia,”Ann Intern Med, 1984, 100(4):535-40.
13. Miller ML, “Heparin-Induced Thrombocytopenia,”Cleve Clin J Med, 1989, 56(5):483-90.
14. Newman PM and Chong BH, “Heparin-Induced Thrombocytopenia: New Evidence for the Dynamic Binding of Purified Anti-PF4-Heparin Antibodies to Platelets and the Resultant Platelet Activation,”Blood, 2000, 96(1)182-7.
15. Amiral J, Wolf M, Marfaing-Koka A, et al, “Characteristics of Antibodies to PF4, IL-8, and NAP-2 Complexed to Heparin in Patients With Heparin-Induced Thrombocytopenia. A study of 187 cases,”Thromb Haemost , 1997, (Suppl):449.
16. Chong BH, Burgess J, and Ismail F, “The Clinical Usefulness of the Platelet Aggregation Test for the Diagnosis of Heparin-Induced Thrombocytopenia,”Thromb Haemost, 1995, 69:344-50.
17. Arepally G, Reynolds C, Tomaski A, et al, “Comparison of PF4/Heparin ELISA Assay With the 14C-Serotonin Release Assay in the Diagnosis of Heparin-Induced Thrombocytopenia,”Am J Clin Pathol, 1995, 104(6):648-54.
18. Amiral J, Bridey F, Wolf M, et al, “Antibodies to Macromolecular Platelet Factor 4-Heparin Complexes in Heparin-Induced Thrombocytopenia: A Study of 44 Cases,”Thromb Haemost, 1995, 73(1):21-8.
19. Warkentin TE, “Heparin-Induced Thrombocytopenia: IgG-Mediated Platelet Activation, Platelet Microparticle Generation, and Altered Procoagulant/Anticoagulant Balance in the Pathogenesis of Thrombosis and Venous Limb Gangrene Complicating Heparin-Induced Thrombocytopenia,”Transfus Med Rev, 1996, 10(4):249-58.
References
Warkentin TE, Chong BH, and Greinacher A, “Heparin-Induced Thrombocytopenia: Toward Consensus,”Thromb Haemost, 1998, 79(1):1-7.
