MDCalc

4Ts Score for Heparin-Induced Thrombocytopenia

Stratifies patients by their probability of having HIT as the cause of thrombocytopenia.

To calculate the percent fall in platelets, use the highest platelet count immediately before the potentially HIT-related decline.

To calculate the number of days, consider the first day of heparin exposure as Day 0. Use the day the platelet count begins to fall after Day 0, not the day the platelet nadir is reached.

Thrombocytopenia
Timing of platelet count fall
Thrombosis or other sequelae
Other causes for thrombocytopenia

Result:

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Advice

This tool should not be the sole basis for care; use clinical judgment and a complete clinical evaluation to guide management.

Management
  • Low probability (score 0–3): HIT is extremely unlikely. 
    • Do not order HIT antibody testing or a functional assay.
    • Evaluate for other causes of thrombocytopenia and consult hematology if needed.
    • Continue or restart heparin if indicated.
  • Intermediate probability (score 4–5): HIT is possible. 
    • Discontinue heparin products and substitute with a non-heparin anticoagulant.
    • Order HIT antibody testing (ELISA); based on the result, determine whether a functional assay (e.g., serotonin release assay) is needed to diagnose HIT.
    • Assess for thrombosis if HIT is diagnosed.
    • Consider hematology consultation. 
  • High probability (score 6–8): HIT is likely. 
    • Discontinue heparin products and substitute with non-heparin anticoagulant.
    • Order HIT antibody testing (ELISA); based on the result, determine if a functional assay (e.g., serotonin release assay) is needed to diagnose HIT.
    • Assess for thrombosis if HIT is diagnosed.
    • Consider hematology consultation.
Critical Actions
  • Consider using the HIT Expert Probability (HEP) Score in conjunction with this tool as an alternative assessment prior to time-consuming HIT antibody testing or empiric substitution of heparin with another anticoagulant; it was developed from expert consensus (26 HIT experts) and demonstrated better interobserver agreement than the 4Ts score.
  • For patients above the screening threshold for HIT, pursue laboratory evaluation and switch to a non-heparin anticoagulant as clinically appropriate.
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