Tahera Iqbal, DO, Pathology Resident
Nam Kyun Ku, MD, Health Sciences Assistant Clinical Professor
Heparin is a commonly-used anticoagulant that is given to patients to prevent the formation of blood clots used in the setting of certain medical condition or medical procedures. Heparin-induced thrombocytopenia (HIT) is a drug-induced thrombocytopenia which occurs due to immune complication of heparin therapy (eg. unfractionated heparin (UFH), low molecular weight (LMW) heparin) and can be potentially fatal. HIT differs from other drug induced thrombocytopenia in two major ways: 1) thrombocytopenia is not usually severe with platelet counts rarely falling below 20,000 uL and 2) It is not commonly associated with bleeding and in fact has an increased risk of thrombosis in both veins and arteries . The mortality rate can be as high as 20% although with early recognition and intervention (cessation of heparin therapy and initiation of non-heparin based anticoagulation), it is reported to be below 2%.
There are two types of HIT which are distinguished by the timing, degree of platelet drop and need for laboratory testing. Type 1 usually presents with a mild transient drop in platelet counts (nadir: 100,000) within the first 2 days after exposure to heparin and it normalizes with continued heparin administration. It is a nonimmune-mediated disorder that occurs due to the direct effect of heparin on platelets causing platelet aggregation. It does not cause any thrombosis and has no clinical significance. Therefore, these patients can be managed without discontinuation of heparin and there is no indication for HIT laboratory testing.
Type II is an immune mediated disorder that is clinically significant which typically occurs 4-10 days after exposure to heparin . It is caused by antibodies to complexes of platelet factor 4 (PF4) and heparin. The complex in turn activates platelets through FcyRIIa receptor and possibly activates endothelial cells which can cause thrombosis, known as heparin induced thrombocytopenia and thrombosis (HITT) . This life-threatening complication can occur in a small percentage of patients regardless of the dose, schedule or the route of administration. However, only a fraction of patients who develop the antibodies to the heparin/complex will develop thrombocytopenia. Although HIT can occur with both LMWH and UFH, it is about 10 times more common to develop it with UFH . Majority of the patients develop HIT after exposure to heparin in 5 to 10 days. It can occur prior to 5 days when the patients had prior exposure to heparin in the prior few weeks or months and already have circulating anti-heparin/PF4 antibodies. In addition, clinically HIT may manifest as skin lesions at the heparin injection site or the patient may experience acute systemic reactions such as fever, chills, dyspnea and chest pain. HIT is managed by prompt withdrawal of heparin and an initiation of a non-heparin anticoagulant.
The evaluation of HIT includes assessing both clinical features and if indicated laboratory testing. The 4T score is a diagnostic tool that is most common used to evaluate the pretest probability of HIT (Table 1). The 4Ts assess evidence of Thrombocytopenia, the Timing of platelet count fall, presence of Thrombosis or any other sequelae and any oTher possible causes of thrombocytopenia .
Table 1: The 4Ts pretest probability of HIT (Adapted from Favaloro et al, American Journal of Hematology)
It is important to perform a clinical assessment because the screening tests can result in high rate of false positive. This is because not all patients with antibodies develop pathologic HIT. If the pretest probability of HIT is low (score 0-3) then HIT can be presumptively excluded and additional HIT antibody is not needed. In fact, if the patients are inappropriately managed by having their therapies altered or their heparin is withdrawn, they may be at increased risk of fatal outcome (ie. life threatening thrombosis). HIT testing may be performed on low 4T score, however, consultation with a hematology expert is strongly recommended (Figure 1). If the 4T score is intermediate (4-5) or high probability (6-8), a presumptive diagnosis of HIT is made with pending results of HIT antibody testing and heparin therapy should be ceased with initiation of an alternate therapy. Immunoassays (ELISA) are used as screening test and they are more sensitive to HIT than functional assays. If the immunoassays are positive or indeterminate, a secondary confirmatory test is performed .
Figure 1: Algorithm for HIT Evaluation (Adapted from Favaloro et al, American Journal of Hematology)
HIT Laboratory Testing
Immunoassays detect the presence of PF4-heparin antibody in the patient's serum. The solid phase ELISA is the gold standard and is the most widely used test to evaluate HIT. This test is performed at UC Davis Medical Center as the screening test. The assay is fast, and easy to interpret, and has a very high sensitivity (91 to 97%), however, they do have a higher incidence of false positive when compared to functional assays. Therefore, if there is low pretest probability of HIT, the screening test is not recommended. ELISA is typically performed in a multi-well plate system and their results are reported in optical density (OD) units. OD level >0.4 is considered a positive result. The level of positivity may be associated with likelihood of pathogenic HIT; higher OD values means higher chance of HIT .
A positive or indeterminate immunoassay is confirmed with a functional assay which test the ability of HIT antibodies from the patient to activate test platelets. This activity strongly correlated with the presence of HIT. Serotonin release assay (SRA) which is considered the gold standard test. The sensitivity and specificity of this test is more than 95%. This test is not performed at UC Davis Medical Center and is a send out test with a turn-around time of 1-3 days. In this assay, reactive donor platelets are radiolabeled with 14C-serotonin and incubated with patient serum plus heparin at therapeutic concentrations. SRA is considered positive when 14C-serotonin is released in the presence of platelet activation which may be caused by binding of HIT antibodies to platelets. This assay is expensive, technically demanding, uses radioactive material and results can be delayed due to lack of availability at most institutions . In a study by Warkentin et al, consisting 1553 patient samples, correlation between the OD in a HIT ELISA was evaluated and it was shown higher OD values had a strong correlation with having SRA positivity and vice versa .
Heparin-induced thrombocytopenia (HIT) is a potentially fatal complication of heparin therapy. Early recognition and intervention are crucial in reducing associated morbidity and mortality. Evaluation for HIT involves both clinical assessment and laboratory tests. Clinical assessment using 4T Score is important in determining probability of HIT. It is not recommended to pursue laboratory tests if pretest probability is low. Laboratory tests are consisted of screening (immunoassay) and confirmatory (functional) assays. Appropriate test utilization following the algorithm (Fig. 1) is recommended.
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