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    4Ts Score for Heparin-Induced Thrombocytopenia

    Differentiates patients with HIT from those with other causes of thrombocytopenia.
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    When to Use
    Pearls/Pitfalls
    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
    +2
    Platelet count fall 30–50% OR platelet nadir 10–19
    +1
    Platelet count fall <30% OR platelet nadir <10
    0
    Clear onset between days 5–10 OR platelet fall ≤1 day (prior heparin exposure within 30 days)
    +2
    Consistent with days 5–10 fall, but not clear; onset after day 10 OR fall ≤1 day (prior heparin exposure 30–100 days ago)
    +1
    Platelet count fall <4 days without recent exposure
    0
    New thrombosis OR skin necrosis; acute systemic reaction post-IV heparin bolus
    +2
    Progressive OR recurrent thrombosis; non-necrotizing skin lesions; suspected thrombosis (not proven)
    +1
    None
    0
    None apparent
    +2
    Possible
    +1
    Definite
    0
    About the Creator
    Dr. Andreas Greinacher
    Content Contributors

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    Next Steps
    Evidence
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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).

    Formula

    Addition of the selected points, see below:

    Facts & Figures

    Category 2 points 1 point 0 points
    Thrombocytopenia Platelet count fall > 50% AND platelet nadir ≥ 20 × 109 L−1 Platelet count fall 30%–50% OR platelet nadir 10–19 × 109 L−1 Platelet count fall < 30% OR platelet nadir < 10 × 109 L−1
    Timing of platelet count fall Clear onset between days 5 and 10 OR platelet fall ≤ 1 day (prior heparin exposure within 30 days) Consistent with days 5–10 fall, but not clear (e.g. missing platelet counts) OR onset after day 10 OR fall ≤ 1 day (prior heparin exposure 30–100 days ago) Platelet count fall < 4 days without recent heparin exposure
    Thrombosis or other sequelae New thrombosis (confirmed) OR skin necrosis at heparin injection sites OR acute systemic reaction after intravenous heparin bolus Progressive or recurrent thrombosis or nonnecrotizing (erythematous) skin lesions or suspected thrombosis (not proven) None
    Other causes for thrombocytopenia None apparent Possible Definite
    Dr. Andreas Greinacher

    About the Creator

    Andreas Greinacher, MD, is the head of the Institute of Immunology and Transfusion Medicine at the University Hospital Greifswald, Germany. He has interests in hereditary and acquired platelet disorders, bridging immuno-hematology and hemostasis. His recent work is on adopting nanotechnology and biophysical methods to investigate platelets and protein changes.

    To view Dr. Andreas Greinacher's publications, visit PubMed

    Content Contributors