4Ts Score for HIT

Heparin-Induced Thrombocytopenia Clinical Decision Support

Total Score

0 / 8

Select all options to see risk

1. Thrombocytopenia (Magnitude of Platelet Drop)

Note: Calculate % drop from the peak post-heparin platelet count, not the pre-heparin baseline. In post-op patients with a biphasic pattern, measure from the rebound peak.

2. Timing of Platelet Count Fall

Note: Timing resets from the rebound peak in post-op patients with a biphasic platelet pattern (initial post-op fall, then rise before HIT-related fall). "Prior exposure" refers to heparin within the past 100 days.

When did the platelet count begin its consistent decline?

Count from first day of heparin exposure — or from the rebound peak in post-op biphasic patterns.

3. Thrombosis or Other Sequelae

Note: Thrombosis in HIT is typically venous (DVT, PE) but arterial events (stroke, limb ischemia) also occur. Skin necrosis typically appears at heparin injection sites due to local platelet activation and microvascular thrombosis.

4. Other Causes for Thrombocytopenia

Note: Common alternative causes: sepsis, DIC, post-cardiac surgery hemodilution, medication-induced (vancomycin, linezolid, quinine), liver disease/hypersplenism, TTP/HUS. Consider all concurrent clinical factors.

Risk Interpretation

0–3: Low probability — NPV ~99.8%; HIT very unlikely

4–5: Intermediate probability — Stop heparin; send antibody testing

6–8: High probability — Discontinue heparin; start alternative anticoagulation

Disclaimer: This tool is intended for clinical decision support only and does not replace clinical judgment, institutional protocols, or specialist consultation. The 4Ts score has a high negative predictive value for low scores but should always be interpreted in the full clinical context. Confirmatory laboratory testing (HIT ELISA, serotonin release assay) is recommended for intermediate and high scores.