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4Ts Score for Heparin-Induced Thrombocytopenia

The 4Ts score estimates pretest probability of heparin-induced thrombocytopenia from Thrombocytopenia, Timing, Thrombosis and oTher causes (0–8). A score of 0–3 is low probability (HIT effectively ruled out), 4–5 intermediate and 6–8 high; test and stop heparin when intermediate or high.

Thrombocytopenia: Platelet fall > 50% and nadir ≥ 20 ×10⁹/L (2); Platelet fall 30–50% or nadir 10–19 ×10⁹/L (1); Platelet fall < 30% or nadir < 10 ×10⁹/L (0)
Timing of platelet fall: Clear onset day 5–10, or fall ≤ 1 day with heparin exposure in past 30 days (2); Consistent with day 5–10 but unclear (missing counts), onset after day 10, or fall ≤ 1 day with heparin 30–100 days ago (1); Fall < 4 days without recent heparin exposure (0)
Thrombosis or other sequelae: New thrombosis (confirmed), skin necrosis at injection site, or acute systemic reaction after IV heparin bolus (2); Progressive or recurrent thrombosis, non-necrotizing skin lesions, or suspected thrombosis not yet proven (1); None (0)
Other causes of thrombocytopenia: None apparent (2); Possible other cause (1); Definite other cause (0)

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Interpretation

Result	Meaning
0–3	Low probability (NPV ~99%)
4–5	Intermediate probability
6–8	High probability

Formula

Sum of 4 items, each 0–2: Thrombocytopenia + Timing + Thrombosis + oTher causes (max 8).

Clinical pearls

The main value of the 4Ts is ruling out HIT: a low score has a very high negative predictive value.
Day 0 is the first day of the most recent heparin exposure; rapid-onset HIT (≤1 day) needs heparin exposure in the prior 100 days.
With intermediate or high scores, stop all heparin (including flushes) and use a non-heparin anticoagulant while awaiting tests.

Examples

4Ts score: platelets fell 60% to nadir 45 on day 7 of heparin, new DVT, no other cause hit probability plt drop 40% day 12, no thrombosis, possible sepsis what is the 4 T score for a patient whose platelets dropped by half on day 6 with no other explanation

FAQ

What 4Ts score rules out HIT?

A score of 0–3 (low probability) has a negative predictive value of about 99%, so HIT testing and heparin cessation are usually not needed.

What do I do with an intermediate 4Ts score?

Stop heparin, start a non-heparin anticoagulant such as argatroban, bivalirudin, fondaparinux or a DOAC, and send an anti-PF4 immunoassay.

Evidence

Lo GK et al. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost 2006;4:759–65.

Hematology & Oncology · Reviewed 2026-10-14 · Methodology