What is the diagnostic approach for Heparin (unfractionated heparin) induced thrombocytopenia (HIT)?

From the Guidelines

The diagnostic approach for heparin-induced thrombocytopenia (HIT) should start with calculating a 4T score, followed by an immunoassay and then a functional assay if necessary, as recommended by the American Society of Hematology 2018 guidelines 1. To diagnose HIT, the following steps should be taken:

• Calculate the 4T score based on the degree of thrombocytopenia, timing of platelet count fall, presence of thrombosis, and absence of other causes of thrombocytopenia.
• If the 4T score is low (<4), HIT is unlikely and further testing may not be necessary 1.
• If the 4T score is intermediate or high, an immunoassay such as ELISA should be performed to detect anti-PF4/heparin antibodies.
• If the immunoassay is positive, a functional assay like serotonin release assay (SRA) or heparin-induced platelet activation (HIPA) test should be performed for confirmation.
• Patients with suspected HIT should have immediate discontinuation of all heparin products while awaiting test results, as recommended by the guidelines 1. The 4T score is a useful tool in estimating the probability of HIT, and the guidelines recommend using it instead of a gestalt approach 1. It's also important to note that different immunoassays and functional assays are available, and the choice of assay may be influenced by diagnostic accuracy, availability, cost, feasibility, and turnaround time 1. In general, the likelihood of HIT increases with a higher 4T score and a higher ELISA optical density (OD), and a functional assay may not be necessary for patients with a high-probability 4T score and very strongly positive immunoassay 1. Another study published in 2016 discussed the use of heparin-induced multi-electrode aggregometry method for HIT testing, but the 2018 guidelines from the American Society of Hematology are more recent and relevant to the current diagnostic approach 1.

From the Research

Diagnostic Approach for Heparin-Induced Thrombocytopenia (HIT)

The diagnostic approach for HIT involves a combination of clinical assessment and laboratory testing. The key elements of this approach are:

• 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 2
• Use of the 4Ts score to calculate the clinical probability of HIT 3, 4, 5
• Laboratory testing, including screening immunoassay and confirmatory functional assay, to detect anti-PF4-heparin antibodies 2, 3, 5, 6
• 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 2

Laboratory Testing

Laboratory testing for HIT includes:

• Screening immunoassay to detect anti-PF4-heparin antibodies 3, 6
• Confirmatory functional assay, such as serotonin release assay or platelet aggregation test, to confirm the diagnosis of HIT 3, 5, 6
• A positive functional assay result confirms the diagnosis of HIT and should be performed to confirm a positive immunoassay result 3

Management of HIT

The management of HIT involves:

• Discontinuation of all forms of heparin 2, 3, 4, 5
• Administration of an alternative form of anticoagulation, such as argatroban, lepirudin, bivalirudin, or fondaparinux, until the platelet count recovers 2, 3, 4, 5, 6
• Avoidance of warfarin therapy until platelet recovery 2
• Use of direct oral anticoagulants in place of warfarin after patients with HIT have responded to alternative parenteral anticoagulants with platelet count recovery 3