Instructions
The 4Ts score assigns 0–2 points in each of four categories (maximum 8):
| Category | 2 points | 1 point | 0 points |
|---|---|---|---|
| Thrombocytopenia | >50% fall OR nadir ≥20 × 10⁹/L | 30–50% fall OR nadir 10–19 × 10⁹/L | <30% fall OR nadir <10 × 10⁹/L |
| Timing of onset | Days 5–10 or ≤1 day if prior heparin within 100 days | >10 days, or timing unclear, or ≤1 day with prior heparin 31–100 days ago | ≤4 days without recent heparin |
| Thrombosis or other sequelae | New thrombosis, skin necrosis, acute systemic reaction | Progressive/recurrent thrombosis, erythematous skin lesions, suspected (not confirmed) | None |
| oTher causes of thrombocytopenia | No other cause | Possible other cause | Definite other cause |
Score Interpretation
- 0–3 (Low probability, ~1%): HIT unlikely; heparin can continue
- 4–5 (Intermediate probability, ~10%): Stop heparin, send PF4/heparin antibody, use alternative anticoagulation
- 6–8 (High probability, ~50%): Stop heparin immediately, use alternative anticoagulant (argatroban, fondaparinux, bivalirudin), send confirmatory testing
When to Use
Appropriate population
Any patient on heparin (UFH or LMWH) with new thrombocytopenia (≥50% platelet fall or nadir <100,000). CKD/HD patients are commonly heparin-exposed; HIT risk is real in dialysis settings. Use before ordering PF4/heparin ELISA — low 4Ts score has high NPV, avoiding unnecessary expensive testing.
Pearls & Pitfalls
High NPV with low score
Low 4Ts (0–3) has very high NPV (~99.8%); can avoid PF4 testing. High 4Ts warrants empirical heparin cessation BEFORE lab confirmation.
Critical pitfalls
- LMWH cross-reacts with HIT antibodies — do NOT switch to LMWH
- Argatroban is renally-dosed; dose-reduce in hepatic failure
- Warfarin is contraindicated in acute HIT (venous limb gangrene risk from protein C depletion)
- Platelet transfusion is generally contraindicated in HIT
Why Use It
The 4Ts score avoids unnecessary PF4 testing in low-probability cases and identifies high-probability cases needing immediate heparin cessation. It is the most widely validated clinical pretest probability tool for HIT and is recommended by both ASH and ACCP guidelines as the first step in HIT evaluation.
4Ts Score
Select the option that best describes each of the four categories. The score and clinical recommendation update automatically.
The 4Ts score is a pretest probability tool, not a diagnostic test. Confirmatory testing (PF4/heparin ELISA, serotonin release assay) is required for definitive diagnosis. Clinical context is essential. For educational reference only. Reference: Lo GK et al., J Thromb Haemost 2006.
Next Steps
- Low (0–3): HIT unlikely; continue heparin; consider other causes of thrombocytopenia
- Intermediate (4–5): Stop all heparin; send PF4/heparin IgG; start non-heparin anticoagulant (argatroban or fondaparinux)
- High (6–8): Stop all heparin immediately; start argatroban or fondaparinux; confirm with SRA (serotonin release assay); Hematology consult
Evidence & References
References
- Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost. 2006;4(4):759–765.
- Cuker A, Arepally GM, Chong BH, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: heparin-induced thrombocytopenia. Blood Adv. 2018;2(22):3360–3392.
