From the Guidelines
Laboratory tests for diagnosing heparin-induced thrombocytopenia (HIT) should include a complete blood count to document thrombocytopenia, calculation of the 4T score, and laboratory confirmation with immunoassays such as PF4/heparin ELISA or functional assays like serotonin release assay (SRA) or heparin-induced platelet activation (HIPA) test, as recommended by the most recent guidelines 1. The initial evaluation should include a complete blood count to document thrombocytopenia (platelet count typically falls by >50% or to <150,000/μL) and assess the timing of thrombocytopenia in relation to heparin administration.
- The 4T score should be calculated to assess pretest probability based on thrombocytopenia timing, thrombosis, alternative causes, and platelet count.
- For laboratory confirmation, immunoassays such as the PF4/heparin ELISA detect antibodies against platelet factor 4-heparin complexes and are highly sensitive but less specific.
- Functional assays like the serotonin release assay (SRA) or heparin-induced platelet activation (HIPA) test measure the ability of patient serum to activate platelets in the presence of heparin and are more specific but less widely available. A positive immunoassay with moderate to high 4T score or a positive functional assay confirms HIT diagnosis, as supported by recent studies 1. Testing should be performed before discontinuing heparin if HIT is suspected, but heparin should be immediately discontinued and alternative anticoagulation started (argatroban, bivalirudin, or fondaparinux) while awaiting results to prevent potentially fatal thrombotic complications, in line with the recommendations from the American Society of Hematology 1 and other guidelines 1.
From the FDA Drug Label
If the platelet count falls below 100,000/mm3 or if recurrent thrombosis develops, promptly discontinue heparin, evaluate for HIT and HITT, and, if necessary, administer an alternative anticoagulant. Obtain platelet counts before and periodically during heparin therapy. Monitor thrombocytopenia of any degree closely
The labs to monitor for Heparin-Induced Thrombocytopenia (HIT) include:
- Platelet count: before and periodically during heparin therapy The key threshold for discontinuing heparin and evaluating for HIT is a platelet count below 100,000/mm3 2.
From the Research
Heparin-Induced Thrombocytopenia Labs
- Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder that occurs following the administration of heparin and is caused by antibodies to platelet factor 4 and heparin 3.
- Diagnosis of HIT is essential to guide treatment strategies using non-heparin anticoagulants and to avoid unwanted and potential fatal thromboembolic complications 3.
- A 4Ts score is recommended for all patients with suspected HIT prior to laboratory testing, and further laboratory testing with a screening immunoassay or confirmatory functional assay is not recommended in individuals with a low 4Ts score 3.
Laboratory Testing for HIT
- A positive functional assay result confirms the diagnosis of HIT and should be performed to confirm a positive immunoassay result 3.
- Documentation of an anti-PF4-heparin antibody is necessary, but is not sufficient to make the diagnosis since antibody formation occurs in a variety of clinical settings without the development of thrombocytopenia or thrombosis 4.
- Accurate diagnosis of HIT is based on the presence of clinical features, including a 50% fall in platelet count, appropriate timing of thrombocytopenia, development of new thrombosis despite thrombocytopenia and heparin therapy, and the absence of a more likely cause of thrombocytopenia 4.
Treatment of HIT
- Once HIT is suspected or confirmed, all forms of heparin should be discontinued and an alternative form of anticoagulation should be administered until the platelet count recovers 4.
- Treatment options include intravenous administration of argatroban, lepirudin, and bivalirudin; subcutaneous administration of fondaparinux has also been described 4.
- Argatroban has been shown to have superior outcomes compared to other anticoagulant drugs used in the management of HIT, with shorter hospitalization length and lower rates of major and minor hemorrhagic events, thromboembolic events, and mortality 5.
- Direct oral anticoagulants (DOACs) have been increasingly investigated for the management of HIT and may overcome the challenges of current therapies, with preliminary results showing promise for their use in acute HIT 6, 7.