About 0.2% of patients exposed to heparin develop heparin-induced thrombocytopenia (HIT)
Heparin-induced thrombocytopenia is an uncommon condition mediated by anti-PF4 platelet-activating antibodies that typically begins 5–14 days after heparin initiation. Overall, greater risk is associated with unfractionated heparin than with low-molecular-weight heparin.1
This acquired hypercoagulable state carries a high risk of venous and arterial thrombosis
The platelet count nadir ranges from 10 to 150 (median: about 60) × 109/L, or a drop of 50% or more.1 As many as 70% of patients with HIT experience thrombosis, most often deep-vein thrombosis, pulmonary embolism or both, but also arterial thrombosis.1
A 4Ts score > 3 should prompt antibody screening and, if positive, a platelet activation assay
A 4Ts score estimates pretest probability of HIT compared with other causes of thrombocytopenia, assessing platelet count, timing, sequelae and causes.2 Screening is usually performed by immunologic assays (e.g., enzyme-linked immunosorbent assay [ELISA]). If positive, a confirmatory platelet activation test (e.g., serotonin-release assay) is required as many patients with a positive ELISA result do not have HIT.
If HIT is suspected, heparin should be stopped and alternative anticoagulation started
Warfarin should be avoided and vitamin K administered if warfarin has already been given, because of the risk of warfarin-associated microthrombosis.3 Factor Xa inhibitors (fondaparinux, apixaban, rivaroxaban) and thrombin inhibitors ( argatroban, bivalirudin, dabigatran) should be considered if the patient needs anticoagulation. Intravenous immunoglobulin may be beneficial in atypical, severe HIT.4 Patients should be referred to a hematologist if possible.
Vaccine-induced immune thrombotic thrombocytopenia (VITT) is an uncommon complication of the SARS-CoV-2 vaccine produced by AstraZeneca (ChAdOx1 nCoV-19) that mimics severe HIT in patients without exposure to heparin
Clinical features of VITT include thrombocytopenia and unusual thrombi, including cerebral venous sinus thrombosis and splanchnic vein thrombosis.5 A 4Ts score, substituting “vaccine” for “heparin,” can be used. Treatment for VITT is similar to that for HIT but emphasizes high-dose intravenous immunoglobulin. Diagnostic testing for VITT antibodies is available in Canada (McMaster Platelet Immunology Laboratory, in Hamilton, Ont.).
Footnotes
Competing interests: Theodore Warkentin reports receiving grants from Instrumentation Laboratory, royalties from Informa (Taylor & Francis), consulting fees from Aspen Global and Ergomed, lecture honoraria from Alexion Canada and Instrumentation Laboratory, and payment for providing expert witness testimony relating to heparin-induced thrombocytopenia (HIT) and non-HIT thrombocytopenic and coagulopathic disorders. Dr. Warkentin has participated on advisory boards for CSL Behring and Octapharma and been a member of a committee on disseminated intravascular coagulation for the International Society on Thrombosis and Haemostasis. Menaka Pai reports receiving royalties from UpToDate and support for attending a conference from Scripps. No other competing interests were declared.
This article has been peer reviewed.
References
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