4Ts Score (HIT)

How to use it

1. 1

   4 domains, each 0–2. Thrombocytopenia severity, timing of fall, thrombosis/sequelae, other causes.

2. 2

   Score ≤3: low probability. Negative HIT ELISA has high NPV — HIT excluded in most cases.

3. 3

   Score ≥6: empirically stop heparin. Switch to argatroban, bivalirudin, or fondaparinux immediately.

When to use it

Platelet fall >50% during heparin exposure

Any heparin (UFH or LMWH, any dose) within 5–10 days.

New thrombosis on heparin

Paradoxical thrombosis while anticoagulated = classic HIT presentation.

Not for routine surveillance

4Ts pre-test probability guides ELISA ordering, not routine platelet monitoring.