cardiology & thrombosisscoring tool

4Ts Score (Heparin-Induced Thrombocytopenia)

The 4Ts score estimates the pretest probability of heparin-induced thrombocytopenia (HIT) based on four clinical criteria: degree of Thrombocytopenia, Timing of platelet fall, presence of Thrombosis, and likelihood of oTher causes. It is the most widely used clinical tool for HIT pre-test probability assessment.

inputs

✓ when to use

Use when HIT is suspected in any patient on heparin (including UFH, LMWH, or heparin flushes) who develops thrombocytopenia (platelet fall >30–50%) or new thrombosis during or shortly after heparin exposure. The 4Ts score determines whether HIT antibody testing is warranted and guides immediate management decisions.

✗ when not to use

The 4Ts is a pre-test probability tool, not a diagnostic test. It performs best as a rule-out tool — a low 4Ts score (<4) has a high negative predictive value (~99.8%) and essentially excludes HIT. However, intermediate and high scores require laboratory confirmation (immunoassay ± functional assay). The 'other causes' criterion is subjective and ICU patients often score 1 here due to multifactorial thrombocytopenia.

clinical pearls

  • The 4Ts score's greatest value is RULING OUT HIT. A score of 0–3 has a negative predictive value >99% — you can confidently continue heparin and look for other causes of thrombocytopenia.
  • In ICU patients, the 4Ts score performs less well because most ICU patients have other plausible causes for thrombocytopenia (sepsis, DIC, medications), making it hard to score >1 on the 'other causes' criterion. Consider a lower threshold for testing in ICU populations.
  • Timing is the most important criterion. Classic HIT presents 5–10 days after heparin initiation. A platelet fall on day 2 of first-ever heparin exposure is almost never HIT (consider HIT only if prior heparin exposure within 30 days, which causes rapid-onset HIT).
  • Remember that HIT can occur with ANY heparin exposure, including LMWH, UFH flushes for line patency, and heparin-coated catheters. Take a thorough heparin exposure history.
  • Do not wait for antibody results before acting on a high-probability 4Ts score. Stop heparin, initiate alternative anticoagulation, and then confirm with testing. HIT carries a 50% thrombosis risk if untreated.

related tools

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