HIT Expert Probability (HEP) Score for Heparin-Induced Thrombocytopenia

HEP Score Calculator
Magnitude of Thrombocytopenia
Timing of Platelet Fall
Nadir Platelet Count
Thrombosis
Other Causes of Thrombocytopenia
Timing of Other Causes
Result: -7
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Pre-test clinical scoring model for HIT based on broad expert opinion.

Why Use

The work-up for HIT is time-consuming and expensive. The HEP score could be used as a tool to avoid initiating work-up for HIT and/or empiric substitution of heparin for another anti-coagulant.

When to Use

The heparin-induced thrombocytopenia expert probability (HEP) score is a tool that can potentially aid in diagnosing patients with suspected HIT, and to avoid expensive HIT workups in some patients. Typical-onset HIT: Platelet count begins to fall within 5-10 hours after starting heparin. Rapid-onset HIT: Platelet count begins to fall within 24 hours of starting heparin, strongly associated with recent (in last 100 days) heparin exposure.

Formula

The HEP Score is calculated by the addition of the selected points: Variable Points Thrombocytopenia Features Magnitude of fall in platelet count (peak platelet count to nadir since heparin) <30% -1 30-50% 1 >50% 3 Timing of platelet count fall for patients in whom typical HIT onset is suspected Fall begins <4 days after heparin exposure -2 Fall begins 4 days after heparin exposure 2 Fall begins 5-10 days after heparin exposure 3 Fall begins 11-14 days after heparin exposure 2 Fall begins >14 days after heparin exposure -1 Timing of platelet count fall for patients with prior heparin exposure (last 100 days) in whom rapid HIT onset is suspected Fall begins <48 hours after heparin re-exposure 2 Fall begins >48 hours after heparin re-exposure -1 Nadir platelet count ≤20 × 10⁹/L -2 >20 × 10⁹/L 2 Thrombosis for patients in whom typical HIT onset is suspected New VTE or ATE ≥4 days after heparin exposure 3 Progression of pre-existing VTE or ATE while receiving heparin 2 None 0 Thrombosis for patients with prior heparin exposure (last 100 days) in whom rapid HIT onset is suspected New VTE or ATE after heparin exposure 3 Progression of pre-existing VTE or ATE while receiving heparin 2 None 0 Skin necrosis at subcutaneous heparin injection sites No 0 Yes 3 Acute systemic reaction after IV heparin bolus No 0 Yes 2 Presence of bleeding, petechiae or extensive bruising No 0 Yes -1 Other Causes of Thrombocytopenia Presence of chronic thrombocytopenic disorder No 0 Yes -1 Newly initiated non-heparin medication known to cause thrombocytopenia No 0 Yes -1 Severe infection No 0 Yes -2 Severe DIC (fibrinogen <100 mg/dL and D-dimer >5 µg/mL) No 0 Yes -2 Indwelling intra-arterial device (e.g. IABP, VAD, ECMO) No 0 Yes -2 Cardiopulmonary bypass within previous 96 hours No 0 Yes -1 No other apparent cause No 0 Yes 3

Pearls / Pitfalls

The HIT Expert Probability (HEP) Score is a tool developed based on broad expert opinion to help clinicians rule out HIT. Eight clinical features potentially important in the diagnosis of HIT identified via literature review were incorporated into a questionnaire and distributed to independent HIT experts that regularly diagnose and treat HIT. Clinicians then weighed each feature on a scale of -3 to 3 with more positive numbers arguing in favor of HIT. Median weights were then incorporated into a pre-test probability model, the HEP score. Score was then prospectively applied to fifty consecutive patients referred to a single reference laboratory for HIT testing. HIT was diagnosed by consensus of three independent HIT expert adjudicators reviewing the clinical information and lab results including polyspecific HIT antibody ELISA and in-house SRA. Study found that a HEP score cut off of 2 would lead to 100% sensitivity and 60% specificity while a cut off of 5 would maximize sensitivity/specificity at 86% and 88%, respectively, in the cohort of patients analyzed. This was superior to the 4Ts for HIT Score . Points to keep in mind: Validation study was very small at 50 patients, highly heterogeneous, and all done at a single large academic health care system. All determinations of HIT were done based on chart review. Score was derived based purely on expert opinion and the confirmatory diagnosis of HIT was also based on the consensus of an expert panel. A subsequent small (47 patients) retrospective study comparing the performance of the 4T’s scoring system vs HEP in suspected HIT patients, using a positive HPA as the gold standard for diagnosis of HIT, found no statistically significant difference in the diagnostic performance of the scores. It also demonstrated only a 70% sensitivity for the diagnosis of HIT at the previously proposed HEP score cut-off of 2. There are no large scale or prospective studies validating the HEP score.

Critical Actions

Consider using the 4Ts scoring system in conjunction with the HEP score as an alternative evaluation tool prior to time-consuming antibody testing for HIT or empiric substitution of heparin for another anti-coagulant. Consider further laboratory evaluation for HIT or switching to a non-heparin derived anti-coagulant in those patients that are above the screening threshold for HIT based on their HEP score. Larger studies of the HEP score are likely necessary prior to broader independent implementation.

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