Thrombosis & Bleeding

Heparin-Induced Thrombocytopenia Probability (4Ts) Calculator

Pretest probability of heparin-induced thrombocytopenia from the platelet fall, its timing, thrombosis and other causes.

Reviewed by Alex G, BSN, RN, CEN, CCRN, CMC, NREMT-P · From the Reference tab of the app

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Formula

Four categories scored 2, 1 or 0 and added (total 0–8); platelets in ×10⁹/L, first day of heparin = day 0
Thrombocytopenia: fall >50% and nadir ≥20 = 2; fall 30–50% or nadir 10–19 = 1; fall <30% or nadir <10 = 0
Timing: clear onset days 5–10, or fall ≤1 day with heparin in the past 30 days = 2; consistent with days 5–10 but not clear, onset after day 10, or fall ≤1 day with heparin 30–100 days ago = 1; fall <4 days without recent exposure = 0
Thrombosis or other sequelae: new confirmed thrombosis, skin necrosis, or acute systemic reaction after an IV heparin bolus = 2; progressive, recurrent or suspected thrombosis, or non-necrotizing skin lesions = 1; none = 0
Other causes of thrombocytopenia: none apparent = 2; possible = 1; definite = 0
Low 0–3, intermediate 4–5, high 6–8

How it is used

• Low probability (0–3): no HIT laboratory testing and no empiric HIT treatment; heparin can continue (ASH 2018).
• Intermediate (4–5): stop all heparin, including flushes, start a non-heparin anticoagulant (prophylactic dose if bleeding risk is high, otherwise therapeutic) and send an immunoassay (ASH 2018).
• High (6–8): stop all heparin, start a therapeutic-dose non-heparin anticoagulant (argatroban, bivalirudin, fondaparinux or a DOAC) and send an immunoassay; confirm a positive result with a functional assay where available (ASH 2018).
• A low score reliably rules HIT out, but intermediate and high scores are often false positives (positive predictive values 14% and 64%) and need laboratory confirmation (Cuker 2012).

Key points

• Do not start warfarin in acute HIT until the platelet count recovers, usually to ≥150 ×10⁹/L (ASH 2018).
• Avoid routine platelet transfusion in acute HIT at average bleeding risk (ASH 2018).

Same formula as the app. Open it in the web app.

Related one-pagers

• Bleeding, Reversal, HIT and DIC4Ts 0–3: HIT excluded; keep heparin and do not test.
• IABP ManagementThrombocytopenia, HIT · Count falls; 50% drop, score the 4Ts · Daily count; stop heparin if HIT likely
• Pulmonary Embolism

Related calculators

• Bleeding Risk on Anticoagulation (HAS-BLED)
• Caprini VTE Risk Score for Surgery (Caprini)
• Padua Prediction Score for VTE (Padua)
• Pulmonary Embolism Severity Index (PESI)
• Simplified PESI (sPESI)
• Stroke Risk in Atrial Fibrillation (CHA₂DS₂-VASc)
• Wells Score for PE (Wells)

For education and exam preparation only. Follow your institution’s protocols. For education and exam preparation only. Verify results independently and follow your institution’s protocols. Not a substitute for clinical judgment.