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

From the Guidelines

Heparin-induced thrombocytopenia (HIT) criteria involve a combination of clinical and laboratory findings, with the 4T score being the recommended clinical assessment tool, and management involves immediate heparin cessation and switching to non-heparin anticoagulants like argatroban, bivalirudin, fondaparinux, or direct oral anticoagulants, as recommended by the American Society of Hematology guidelines 1. The diagnosis requires a significant drop in platelet count (typically >50% from baseline) occurring 5-10 days after starting heparin therapy (or sooner with recent heparin exposure). The 4T score evaluates Thrombocytopenia severity, Timing of platelet fall, presence of Thrombosis, and absence of oTher causes of thrombocytopenia. A low 4T score (<4) effectively rules out HIT, while intermediate or high scores warrant laboratory testing, including immunoassays (PF4/heparin ELISA) and functional assays (serotonin release assay) 1. Definitive diagnosis requires both clinical suspicion and positive laboratory tests. This approach is critical because HIT paradoxically increases thrombosis risk despite causing thrombocytopenia, due to antibodies forming against platelet factor 4-heparin complexes, which activate platelets and trigger the coagulation cascade. Recent studies have also explored the use of direct oral anticoagulants (DOACs) as alternative anticoagulants in cases with confirmed or suspected HIT, with some studies showing they may be a safe and effective alternative in select cases 1. However, the use of DOACs in HIT is still not widely accepted and requires further study. In summary, the diagnosis and management of HIT require a combination of clinical and laboratory findings, and the use of non-heparin anticoagulants is critical to prevent further thrombosis and bleeding complications, as supported by the most recent guidelines 1.

Some key points to consider in the diagnosis and management of HIT include:

• The 4T score is a useful tool for estimating the probability of HIT, but it should be used in conjunction with laboratory tests and clinical suspicion 1.
• Immunoassays and functional assays are available for laboratory testing, but they have different specificities and sensitivities, and the choice of assay may depend on the clinical context and availability 1.
• The management of HIT involves immediate heparin cessation and switching to non-heparin anticoagulants, with the choice of anticoagulant depending on the clinical context and patient factors 1.
• Direct oral anticoagulants (DOACs) may be a safe and effective alternative in select cases, but their use is still not widely accepted and requires further study 1.

Overall, the diagnosis and management of HIT require a careful and individualized approach, taking into account the clinical context, laboratory results, and patient factors, as recommended by the most recent guidelines 1.

From the FDA Drug Label

Patients with HITTS also had an arterial or venous thrombosis documented by appropriate imaging techniques or supported by clinical evidence such as acute myocardial infarction, stroke, pulmonary embolism, or other clinical indications of vascular occlusion. HIT/HITTS was defined by a fall in platelet count to less than 100,000/µL or a 50% decrease in platelets after the initiation of heparin therapy with no apparent explanation other than HIT

The HIT criteria include a fall in platelet count to less than 100,000/µL or a 50% decrease in platelets after the initiation of heparin therapy with no apparent explanation other than HIT, often accompanied by arterial or venous thrombosis documented by imaging techniques or clinical evidence such as acute myocardial infarction, stroke, or pulmonary embolism 2.

From the Research

HIT Criteria

The HIT criteria, also known as the "4 Ts", are used to diagnose heparin-induced thrombocytopenia (HIT) and include:

• Thrombocytopenia: a drop in platelet count to less than 50% of the basal level [(3,4,5,6)]
• Timing of platelet count fall: the platelet count begins to drop within 5 to 10 days of starting heparin [(3,5,6)]
• Thrombosis or other complications: the presence of thrombotic complications, such as deep vein thrombosis (DVT) or pulmonary embolism [(3,4,5,6)]
• Other causes for thrombocytopenia: the exclusion of other causes of thrombocytopenia, such as sepsis or drug-induced thrombocytopenia [(3,4,5,6)]

Diagnosis of HIT

Diagnosis of HIT can be established using an HIT antibody test, and the gold standard in diagnosing HIT is the 14C-serotonin release assay (14C-SRA) assay 3. A positive functional assay result confirms the diagnosis of HIT and should be performed to confirm a positive immunoassay result 4.

Treatment of HIT

Treatment of HIT consists of utilizing alternative, nonheparin anticoagulants like lepirudin, argatroban, bivalirudin, or fondaparinux [(3,4,5,7)]. Each of these agents should be individually formulated based on the patient and the presence/absence of liver or renal failure 3. Direct oral anticoagulants (DOACs) may be used in place of warfarin after patients with HIT have responded to alternative parenteral anticoagulants with platelet count recovery [(4,7)].