4T Score Calculator for Heparin-Induced Thrombocytopenia (HIT)

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Use the highest pre-heparin platelet count as the baseline. A nadir <10x10⁹/L typically points to another cause.
Day 0 is the first day of heparin exposure. Rapid-onset HIT (onset day 1) occurs only if the patient had heparin within the preceding 100 days.
New or worsening thrombosis, skin necrosis at heparin injection sites, and acute systemic reactions after IV heparin bolus all count here.
Common alternatives include sepsis, post-cardiac surgery thrombocytopenia, other medications (quinine, vancomycin, linezolid), and dilutional causes.
4T ScoreHigh probability
6

Sum of all four criteria (0-8)

Risk CategoryHigh
Pre-Test Probability~64%
Thrombocytopenia points2
Timing points2
Thrombosis points1
Other causes points1
6 pts
Low<3Intermediate3-5High5+

4T Score 6/8 - High pre-test probability of HIT (~64%)

  • A high 4T score makes HIT very likely. Heparin should be discontinued immediately and replaced with a non-heparin anticoagulant such as argatroban, bivalirudin, or fondaparinux.
  • Order both a HIT antibody ELISA and a confirmatory functional assay (serotonin release assay or HIPA).
  • Avoid warfarin until platelets have recovered to avoid warfarin-associated skin necrosis, which can occur in the setting of active HIT.

Next stepThis tool provides a clinical pre-test estimate only. All management decisions should be made by a licensed clinician using the full clinical context and laboratory findings.

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