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    4Ts Score for Heparin-Induced Thrombocytopenia

    Differentiates patients with HIT from those with other causes of thrombocytopenia.
    When to Use
    Why Use
    • Consider using the 4Ts scoring system to stratify patients’ risk for HIT in patients with thrombocytopenia who are currently or were recently on heparin derived agents.
    • Patients falling into the low-risk category frequently do not need further testing for HIT.
    • Consider further laboratory evaluation for HIT or switching to a non-heparin derived anti-coagulant in those patients in the intermediate or high-risk groups.

    The 4Ts for the diagnosis of heparin-induced thrombocytopenia (HIT) is a tool developed to help clinicians rule out HIT in patients who develop thrombocytopenia in the clinical setting.

    • Included patients being evaluated for thrombocytopenia or suspected HIT in two clinical settings: inpatients at Hamilton General Hospital (HGH) in Canada and various clinicians in a variety of healthcare settings in Germany and Austria.
    • The study prospectively applied previously developed scoring systems using various clinical features of HIT.
    • Scoring system consisted of four criteria, each of which was worth 0, 1, or 2 points.
    • Study used ≤ 3 points to define low probability group (≤5%) for HIT, 4-5 points for intermediate and 6-8 points for high.
    • Gold standard for diagnosis of HIT was either:
      • A platelet serotonin release assay (SRA) and a PF4/polyanion-enzyme immunoassay (EIA) with ≥ 50% serotonin release and positive EIA, OR
      • A positive heparin-induced platelet activation (HIPA) test in at least three of four donor platelets.

    Points to keep in mind:

    • Scoring system criteria for HIT was slightly different between the two main sites.
    • The study used two different tests to diagnose HIT, depending on the site.
    • One of the four components of their scoring system, other causes for thrombocytopenia, is subjective.
    • Results between the two sites in the intermediate and high probability groups were statistically different.

    A subsequent review and meta-analysis of the 4Ts scoring system for HIT found that patients in the low-risk group had a negative predictive value of 0.998, irrespective of type of clinician, prevalence of HIT, or patient population.

    Consider using the 4Ts scoring system as an alternative evaluation tool prior to time-consuming antibody testing for HIT or empiric substitution of heparin for another anti-coagulant.

    Platelet count fall >50% AND platelet nadir ≥20
    Platelet count fall 30–50% OR platelet nadir 10–19
    Platelet count fall <30% OR platelet nadir <10
    Clear onset between days 5–10 OR platelet fall ≤1 day (prior heparin exposure within 30 days)
    Consistent with days 5–10 fall, but not clear; onset after day 10 OR fall ≤1 day (prior heparin exposure 30–100 days ago)
    Platelet count fall <4 days without recent exposure
    New thrombosis OR skin necrosis; acute systemic reaction post-IV heparin bolus
    Progressive OR recurrent thrombosis; non-necrotizing skin lesions; suspected thrombosis (not proven)
    None apparent


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    Critical Actions

    • ≤3 points: low probability for HIT (≤5% in original study, <1% in meta-analysis).
    • 4-5 points: intermediate probability (~14% probability of HIT).
    • 6-8 points: high probability (~64% probability of HIT).
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    About the Creator
    Dr. Andreas Greinacher
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