4Ts Score for HIT

Estimates the clinical pretest probability of heparin-induced thrombocytopenia (HIT) from four domains (thrombocytopenia, timing, thrombosis, other causes), each scored 0–2, to guide heparin cessation and confirmatory immunoassay/functional testing.

Thrombocytopenia (severity)

2 — Platelet count fall > 50% AND nadir ≥ 20 × 10⁹/L

1 — Platelet count fall 30–50% OR nadir 10–19 × 10⁹/L

0 — Platelet count fall < 30% OR nadir < 10 × 10⁹/L

Timing of platelet count fall

2 — Clear onset between days 5–10, OR ≤ 1 day with prior heparin exposure within 30 days

1 — Consistent with days 5–10 but unclear (e.g., missing counts), OR onset after day 10, OR ≤ 1 day with prior heparin 30–100 days ago

0 — Platelet fall < 4 days without recent heparin exposure

Thrombosis or other sequelae

2 — Confirmed new thrombosis, skin necrosis at injection site, or anaphylactoid reaction after IV unfractionated heparin bolus

1 — Progressive / recurrent thrombosis, non-necrotizing (erythematous) skin lesions, or suspected (not proven) thrombosis

0 — None

Other causes of thrombocytopenia

2 — None apparent

1 — Possible other cause

0 — Definite other cause