4T Score (HIT - Heparin-Induced Thrombocytopenia)

The calculator for the 4T Score quantifies the clinical probability of heparin-induced thrombocytopenia (HIT) before laboratory confirmation is available. HIT is a potentially life-threatening immune-mediated reaction in which antibodies against platelet factor 4 (PF4)–heparin complexes cause paradoxical platelet activation, leading to thrombocytopenia and a prothrombotic state that can produce devastating arterial and venous thromboembolism despite — or because of — ongoing heparin therapy.

The Four Clinical Criteria of the 4T Score

Each criterion is scored 0, 1, or 2 points, for a maximum total of 8. The four domains are:

Thrombocytopenia magnitude  |  Timing of platelet fall  |  Thrombosis or other sequelae  |  oTher causes of thrombocytopenia

The timing criterion is the most diagnostically specific: a platelet fall beginning 5–10 days after heparin initiation (or within 1 day in re-exposure) scores 2 points and strongly suggests HIT. Falls occurring outside this window are less specific. The bleeding & anticoagulation calculators category includes related tools for INR, platelet dosing, and hemorrhage risk scoring.

Score Interpretation and Clinical Action

The 4T Score stratifies patients into three probability categories with markedly different management implications:

0–3 pts: Low probability (~5% HIT)  |  4–5 pts: Intermediate (~14%)  |  6–8 pts: High (~64%)

A low score (0–3) has a negative predictive value exceeding 99% — HIT can be safely excluded without laboratory testing, and heparin may be continued. An intermediate or high score mandates immediate heparin cessation, initiation of an alternative anticoagulant, and urgent laboratory confirmation with a functional assay (serotonin release assay) or immunoassay (anti-PF4/heparin ELISA). Do not wait for lab results before acting on a high clinical score.

Why Warfarin Is Contraindicated in Acute HIT

Warfarin is absolutely contraindicated in acute HIT due to its inhibition of protein C, a natural anticoagulant. In the context of HIT's existing hypercoagulable state, warfarin-induced protein C depletion can trigger venous limb gangrene and skin necrosis. Alternative anticoagulants — argatroban, bivalirudin, fondaparinux, or direct oral anticoagulants — must be used until platelet counts recover above 150 × 10⁹/L before any transition to warfarin is considered. Use this online calculator alongside current institutional HIT protocols.

Limitations and Interrater Variability

The 4T Score is a clinical decision support tool, not a diagnostic standard. Its primary limitation is moderate interrater variability — the timing and thrombocytopenia criteria require clinical judgment that differs between providers. Studies show κ values of 0.40–0.60 for the timing criterion. The score performs best when applied by experienced clinicians familiar with HIT. A low score is highly reliable for ruling out HIT; intermediate and high scores require laboratory confirmation before treatment decisions are finalized. The INR calculator and platelet calculator provide supplementary data for managing anticoagulation in these patients.