4Ts Score Calculator (HIT)

Health

The 4Ts score estimates how likely a drop in platelets is caused by heparin-induced thrombocytopenia (HIT). It scores four features — the degree of Thrombocytopenia, the Timing of the platelet fall, the presence of Thrombosis, and whether oTher causes are present — from 0 to 2 each, for a total of 0 to 8. The total places a patient in a low, intermediate or high pre-test probability of HIT.

Formula

4Ts = Thrombocytopenia (0-2) + Timing (0-2) + Thrombosis (0-2) + oTher causes (0-2). 0-3 low, 4-5 intermediate, 6-8 high probability.
  • Each of the four Ts is scored 0, 1 or 2, giving a total from 0 to 8.
  • A total of 0-3 is low probability of HIT, 4-5 is intermediate, and 6-8 is high.
  • A low score (0-3) has a high negative predictive value, so HIT is unlikely and antibody testing is often unnecessary.
  • Intermediate or high scores warrant stopping heparin and ordering an anti-PF4/heparin antibody test.
  • The 4Ts is a clinical pre-test tool, not a diagnosis; confirmatory laboratory testing is still required.

Example Calculation

Inputs
  • Thrombocytopenia (platelet fall): 2
  • Timing of platelet fall: 2
  • Thrombosis or other sequelae: 1
  • oTher causes of thrombocytopenia: 1

A patient with a >50% platelet fall (2), onset on day 7 (2), progressive thrombosis (1) and a possible other cause (1) scores 2 + 2 + 1 + 1 = 6 out of 8 — a high pre-test probability of HIT, so heparin should be stopped and antibody testing ordered.

Frequently asked questions

What does the 4Ts score assess?
It estimates the pre-test probability that a fall in platelets is due to heparin-induced thrombocytopenia (HIT), based on four clinical features each scored 0 to 2.
What are the four Ts?
Thrombocytopenia (how far platelets fell), Timing of the fall relative to heparin exposure, Thrombosis or other sequelae, and whether oTher causes of low platelets are present.
How do I interpret the total?
A score of 0-3 means low probability, 4-5 intermediate and 6-8 high. A low score makes HIT very unlikely and usually rules out the need for antibody testing.
What should I do with an intermediate or high score?
Stop all heparin, start a non-heparin anticoagulant if needed, and send an anti-PF4/heparin antibody (ELISA) test, confirmed by a functional assay.
Does a high 4Ts score diagnose HIT?
No. It only raises the probability. Diagnosis still depends on laboratory antibody and functional testing alongside the clinical picture.