4Ts Score (HIT)

Why use it

Standardizes clinical assessment of HIT probability. High negative predictive value at low scores (< 4). Guides decision on stopping heparin and ordering confirmatory tests (SRA, ELISA). Avoids unnecessary alternative anticoagulation in low-risk patients.

About

The 4Ts scoring system estimates the pre-test probability of heparin-induced thrombocytopenia (HIT). It evaluates four domains: Thrombocytopenia (degree and timing), Timing of onset, Thrombosis or other sequelae, and oTher causes of thrombocytopenia. The score guides decisions on heparin discontinuation and laboratory testing.

When to use

• Platelet drop > 50% or new thrombocytopenia in heparin-exposed patient
• New thrombosis in a patient on heparin
• Before ordering HIT antibody testing

Formula

Pearls & pitfalls

• 4Ts score 0-3 has > 99% NPV — effectively rules out HIT.
• Intermediate and high scores require BOTH stopping heparin AND starting an alternative anticoagulant.
• Do NOT wait for lab confirmation to discontinue heparin in intermediate/high probability.
• Alternative anticoagulants for HIT: argatroban (hepatic clearance) or bivalirudin (renal clearance).
• HIT typically occurs 5-10 days after heparin exposure (or sooner if prior exposure within 100 days).
• Do NOT give warfarin until platelets recover — risk of venous limb gangrene/skin necrosis.

Next steps

Score 0-3: Low probability (NPV > 99%) — HIT very unlikely, continue heparin if clinically indicated. Score 4-5: Intermediate — stop heparin, start alternative anticoagulant, send HIT antibody testing. Score 6-8: High probability — stop heparin immediately, start alternative anticoagulant, send HIT testing.

Evidence

Derived by Lo et al. (J Thromb Haemost, 2006). Validated in multiple cohorts. Systematic review (Cuker et al., Blood 2012) confirmed NPV > 99% for scores 0-3. ASH 2018 guidelines recommend 4Ts as initial assessment tool.

References

1. Lo GK et al. Evaluation of pretest clinical score (4 Ts) for the diagnosis of heparin-induced thrombocytopenia. J Thromb Haemost. 2006;4(4):759-765. — link
2. Cuker A et al. The HIT Expert Probability (HEP) Score: a novel pre-test probability model for heparin-induced thrombocytopenia. J Thromb Haemost. 2010;8(12):2642-2650. — link