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]

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.

• Thrmobocytopenia: 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."