Heparin-induced thrombocytopenia diagnostic criteria

Heparin-induced thrombocytopenia
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Shyam Patel [2]

overview

The diagnostic workup for HIT begins with calculation of the pre-test probability of HIT. The 4T score can be used to determine the likelihood of HIT and whether to proceed with further diagnostic evaluation. The 4T score includes factors such as thrombocytopenia, timing, thrombosis, and other explanations. If the 4T score is intermediate or high, further diagnostic workup should be pursed and includes assessment for clinical and laboratory criteria, including the anti-PF4 IgG optical density. A high titer of antibodies to PF4-heparin is suggestive of a diagnosis of HIT.

Diagnostic Criteria

The 4T score

The current diagnostic algorithm for HIT involves calculation of the pre-test probability using the 4T score. This scoring system is used when a patient is found to have thrombocytopenia or thrombosis after heparin exposure. It is calculated before the heparin-PF4 IgG antibody is ordered, and it offers a cost-effective and efficient way for determining the likelihood of HIT.

• Thrombocytopenia: 2 points are assigned to platelet count fall greater than 50% and platelet nadir greater than 20,000 per microliter. 1 point is assigned to platelet count fall 30-50% or platelet nadir 10,000-19,000 per microliter. No points are assigned to platelet count fall less than 30% or platelet nadir less than 10,000 per microliter.
• Thrombosis: 2 points are assigned to new confirmed thrombosis or skin necrosis at injection sites or acute systemic reaction after IV heparin bolus. 1 point is assigned to progressive or recurrent skin lesions or suspected thrombosis (without proof of thrombosis. No points are assigned if there is no thrombosis.
• Timing: 2 points are assigned if the platelet count fall clearly occurs between days 5 and 10, or if the platelet count fall occurs in less than 1 day if a patient had recent heparin exposure within the past 30 days. 1 point is assigned if the platelet count is likely to have fallen between days 5-10 (there may be missing platelet count checks), or platelet count fall after day 10, or platelet count fall within 1 day in the setting of prior heparin exposure 30-100 days ago. No points are assigned if platelet count fall occurs within 4 days in the setting of no heparin exposure.
• oTher: 2 points are assigned if there is no apparent alternative explanation of thrombocytopenia. 1 point is assigned if there is a possible alternative explanation of thrombocytopenia. No points are assigned if there is a definite alternative explanation of thrombocytopenia.

The range of scores is from 0 to 8. The points from each of the 4 categories is summed in order to obtain the final score, as follows:

• 0 to 3 points: low probability of HIT
• 4 to 5 points: intermediate probability of HIT
• 6 to 8 points: high probability of HIT

The American Society of Hematology, as part of the Choosing Wisely program, states:

"Don’t test or treat for suspected heparin-induced thrombocytopenia (HIT) in patients with a low pre-test probability of HIT."

Clinical and Laboratory Criteria

There are no well-established formal criteria for the diagnosis of HIT. However, there are clinical and laboratory criteria which can strongly suggest a diagnosis of HIT.

Clinical criteria for a diagnosis of HIT include:

• Development of thrombocytopenia and/or thrombosis in the setting of recent heparin exposure
• Absence of other causes of thrombocytopenia
• Timing of thrombocytopenia (i.e. 5-14 days is most suggestive)

Laboratory criteria for diagnosis of HIT include:

• Thrombocytopenia (i.e. a platelet count drop by > 50% with nadir > 20,000 per microliter is most suggestive)
• Presence of heparin-PF4 IgG antibodies via enzyme-linked immunoassay: A high PF4 IgG optical density is more suggestive of HIT than a low optical density. If the optical density is more than 2, a diagnosis of HIT is very likely. This should be confirmed via the functional assay.
• Presence of a positive functional assay (i.e. C14-serotonin release assay or heparin-induced platelet aggregation assay)