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.
Index of Tests
Heparin-Induced Thrombocytopenia [CO006400]
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
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
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
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.
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,
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,
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,
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,
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,
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.
Warkentin TE, Chong BH, and Greinacher A, "Heparin-Induced Thrombocytopenia:
Toward Consensus,"Thromb Haemost, 1998, 79(1):1-7.