Heparin-induced thrombocytopenia (HIT) is an immune-mediated adverse effect of heparin treatment. On exposure to heparin, some patients form antibodies that target complexes of platelet factor 4 (PF4) and heparin. Only a subset of patients with detectable antibodies develop HIT syndrome. Although HIT is a thrombocytopenic disorder, it results in a prothrombotic condition of platelet activation that can lead to life-threatening complications, including pulmonary embolism, limb necrosis, stroke, and acute myocardial infarction. Without treatment, up to 50% of patients can experience thrombotic events. Diagnosis of HIT is based on clinical assessment and laboratory results. Primary laboratory tests for HIT include immunologic assays, such as an enzyme-linked immunosorbent assay (ELISA), and functional, platelet-activation assays, such as the serotonin release assay (SRA). It is recommended that a clinical scoring system, such as the 4Ts system, be used to determine the pretest probability of HIT in a patient before laboratory testing is performed.
Quick Answers for Clinicians
Risk factors for heparin-induced thrombocytopenia (HIT) include the duration of heparin treatment, type of heparin administered (unfractionated heparin [UFH] versus low-molecular-weight heparin [LMWH]), dose, patient population, and patient sex (females are at greater risk of developing HIT than males). Of the two heparin types, UFH is associated with greater risk (10-fold higher) than is LMWH. Patients undergoing major surgery and trauma patients are at increased risk for HIT compared with medical patients (ie, those undergoing minor surgery or general medical treatment).
The American Society of Hematology (ASH) recommends the 4Ts clinical scoring system, which is widely used to assess pretest probability for heparin-induced thrombocytopenia (HIT). This system takes into account the severity and timing of thrombocytopenia, the presence of thrombosis or other complication(s), and the likelihood of another cause of thrombocytopenia. Laboratory testing is typically not performed in patients with low scores (unless missing information could affect score reliability) due to the 4Ts system’s high negative predictive value for HIT. Laboratory testing is performed in patients with intermediate or high 4Ts scores due to the system’s poor positive predictive value. In patients who undergo laboratory testing, the combination of the clinical score and test results is used to determine the likelihood of HIT.
In patients with an intermediate or high pretest probability of HIT, an enzyme-linked immunosorbent assay (ELISA) to detect heparin-platelet factor 4 (PF4) immunoglobulin G (IgG) antibodies is the first diagnostic step. If the ELISA is positive, testing with a functional assay, such as the serotonin release assay (SRA), may be helpful to confirm the diagnosis of HIT.
Although vaccine-induced immune thrombotic thrombocytopenia (VITT) is distinct from heparin-induced thrombocytopenia (HIT), VITT is similarly associated with platelet-activating antibodies against platelet factor 4 (PF4), and testing for the two conditions overlaps. When VITT is suspected (severe symptoms typically occur 4-42 days postvaccination ), initial laboratory testing includes :
- HIT (anti-PF4) antibody detection by enzyme-linked immunosorbent assay (ELISA)
- CBC with platelet count and peripheral smear
- Prothrombin time (PT) test
- Activated partial thromboplastin time (aPTT) test
- Fibrinogen test
- D-dimer test
Specimens for antibody testing should be collected before initiation of intravenous immunoglobulin (IVIG) therapy. One or more functional assays (eg, serotonin release assay) may be used as a follow-up to a positive HIT antibody result.
Indications for Testing
- Diagnose patients with thrombocytopenia after exposure to heparin, particularly those with an intermediate or high probability of HIT based on a clinical scoring system or risk factors
- Guide clinical decision-making in patients with a current or previous diagnosis of HIT
HIT is a clinicopathologic diagnosis that requires combined evaluation of clinical examination and laboratory test results. Use of a clinical scoring system, such as the 4Ts system, is recommended to establish the need for diagnostic laboratory testing in HIT (see table below). Clinical scoring can determine which patients do not need further testing, thereby avoiding the use of expensive and potentially risky nonheparin anticoagulants (such as argatroban) while awaiting test results for these patients.
|Scorea||Category or Variable||Total Score|
|Thrombocytopenia||Timing of Onset||Thrombosis||Other Causes of Thrombocytopenia|
|2||Platelet count decreases >50%
Platelet nadir ≥20 x 109/L
|5-10 days after start of heparin, or
≤1 day (previous heparin exposure within 30 days)
|New thrombosis, or skin necrosis at heparin injection sites, or acute systemic reaction after IV heparin||No other apparent cause||6-8 (high)|
|1||Platelet count decreases 30-50%
Platelet nadir 10-19 x 109/L
|>10 days after start of heparin, or time frame of onset is unclear||Progressive or recurrent thrombosis||Possible other cause||4-5 (intermediate)|
|0||Platelet count decreases <30%
Platelet nadir <10 x 109/L
|≤4 days after start of heparin, with no recent heparin exposure||No thrombosis||Definite other cause||0-3 (low)|
aA score is assigned for each category or variable, and the total 4Ts score (far right column) is the sum of all scores.
HIT Risk by Underlying Medical Condition for Patients Receiving UFH
|1-5%||Patients undergoing cardiac or orthopedic surgerya|
|0.1-1%||Medical or obstetric patients|
HIT Risk by Underlying Medical Condition and Type of Heparin
|High risk (>1.0%)||Patients treated with UFH (or a combination of UFH and LMWH, or UFH and fondparinux) following major surgery or major trauma|
|Intermediate risk (0.1-1.0%)||
Medical or obstetric patients treated with UFH
Patients treated with LMWH following major surgery or major trauma
|Low risk (<0.1%)||
Medical or obstetric patients treated with LMWH
Patients treated with LMWH after minor surgery or minor trauma
Patients treated with fondparinux
LMWH, low-molecular-weight heparin; UFH, unfractionated heparin
Baseline and serial platelet counts are recommended in patients with a HIT risk of at least 0.1% and are used to determine clinical pretest probability. ASH recommends evaluation for HIT using serial platelet counts every 2-3 days for intermediate-risk patients and at least every other day for high-risk patients, beginning on the day of heparin initiation if the patient has been treated with heparin in the preceding 30 days, or on day 4 after heparin exposure in patients who have not received heparin in the previous 30 days. Platelet-count monitoring should continue until day 14 or until cessation of heparin treatment, whichever comes first. For patients estimated to have a low risk of HIT, monitoring of platelet counts is not recommended.
The platelet-count decrease in HIT generally occurs over a period of 1-3 days and is evaluated in comparison with the highest platelet count obtained after heparin treatment began. Thrombocytopenia in HIT typically manifests 5-10 days after heparin is started, but rapid-onset and delayed-onset presentations are also possible. Rapid onset of thrombocytopenia may occur in patients treated with heparin within the past 30 days.
Immunologic assays such as ELISAs are highly sensitive for HIT-related antibodies and are therefore helpful to exclude a HIT diagnosis in conjunction with pretest probability. ELISA immunoglobulin G (IgG) testing has a high negative predictive value. HIT is very unlikely in patients with a negative antibody test. Using an ELISA that measures only clinically relevant IgG antibodies but not IgA and IgM antibodies can improve the specificity and positive predictive value of the test. Higher optical density (OD) values in an ELISA are associated with an increased likelihood of positive functional assay results and clinical HIT; however, OD values vary by assay. Because not all patients with a positive ELISA will have HIT, functional assays are useful to further evaluate certain samples with positive ELISA results.
The SRA is considered the gold standard test for HIT diagnosis because of its high sensitivity and specificity. The test measures the serotonin released from platelets, which correlates with the degree of platelet activation induced by the PF4-heparin antibody complex. SRAs can sometimes be falsely positive due to non-HIT-related platelet-activating antibodies; performing the test in conjunction with an ELISA helps to avoid false-positive SRA results.
Platelet Count Monitoring
Platelet counts serve as a surrogate measure for ongoing platelet activation. Platelet count monitoring is useful to guide clinical decision-making after a confirmed diagnosis of HIT, both to evaluate patients for platelet count recovery and because vitamin K antagonists such as warfarin should not be given until the platelet count is >150,000/µL.
Functional and Immunologic Assays
HIT tests (both functional and immunologic assays) are used to evaluate patients with a history of HIT who may be reexposed to heparin (eg, those who need cardiac surgery); these assays can be used to determine whether HIT antibodies are still present before the use of heparin. However, heparin exposure is generally to be avoided in patients with a history of HIT unless warranted by a specific clinical situation in which alternative anticoagulation is considered inferior to heparin.
ARUP Laboratory Tests
Semi-Quantitative Enzyme-Linked Immunosorbent Assay
Semi-Quantitative Enzyme-Linked Immunosorbent Assay/Serotonin Release Assay
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