What are the criteria for Heparin-Induced Thrombocytopenia (HIT)?

From the Guidelines

Heparin-induced thrombocytopenia (HIT) is diagnosed based on both clinical and laboratory criteria, with a significant drop in platelet count (typically >50% from baseline) and a platelet count < 100 G.L being key diagnostic factors, as noted in the most recent study 1. The clinical criteria for HIT include:

• A significant drop in platelet count (typically >50% from baseline) occurring 5-10 days after starting heparin therapy (or sooner in patients with previous heparin exposure)
• New thrombosis while on heparin
• Skin lesions at injection sites
• Acute systemic reactions after heparin administration Laboratory confirmation involves testing for HIT antibodies, with functional assays like the serotonin release assay being more specific than immunoassays such as the PF4-heparin ELISA, as discussed in 1 and 1. The 4T score is a useful clinical tool that evaluates the probability of HIT based on the degree of Thrombocytopenia, Timing of platelet count fall, presence of Thrombosis, and absence of oTher causes of thrombocytopenia, as mentioned in 1. When HIT is suspected, all heparin products should be immediately discontinued and replaced with non-heparin anticoagulants like argatroban, bivalirudin, fondaparinux, or direct oral anticoagulants, as HIT paradoxically increases thrombotic risk despite low platelet counts, as recommended in 1. Some key points to consider in the diagnosis and management of HIT include:
• The timing of thrombocytopenia in relation to heparin administration, with a typical delay of 5-14 days, as noted in 1
• The rarity of hemorrhagic manifestations and the frequency of venous and/or arterial thrombotic events, as discussed in 1
• The importance of discontinuing heparin therapy and switching to a non-heparin anticoagulant, as recommended in 1 and 1
• The potential use of direct oral anticoagulants (DOACs) as an alternative to traditional non-heparin anticoagulants, as mentioned in 1

From the Research

Criteria for HIT

The criteria for diagnosing Heparin-induced thrombocytopenia (HIT) include:

• A high index of clinical suspicion 2
• Accurate clinical scoring assessment using the 4Ts 2, 3
• Appropriate stepwise laboratory testing to rule out HIT or establish the diagnosis 2, 3
• The presence of antibodies to platelet factor 4 (PF4) and heparin 4, 3
• A median platelet count nadir of approximately 50 to 60 x 10(9) platelets/L 4
• The timing of the thrombocytopenia in relation to the initiation of heparin therapy, with the platelet count beginning to drop within 5 to 10 days of starting heparin 4

Clinical Scoring Assessment

The 4Ts score is recommended for all patients with suspected HIT prior to laboratory testing 3. The 4Ts score includes:

• Thrombocytopenia
• Timing of thrombocytopenia
• Thrombosis
• Other causes of thrombocytopenia

Laboratory Testing

Laboratory testing for HIT includes:

• Immunoassays to detect antibodies based on their binding properties 5
• Functional assays to detect antibodies based on their platelet-activating properties 5
• A positive functional assay result confirms the diagnosis of HIT 3