4Ts Score Calculator — Heparin-Induced Thrombocytopenia (HIT)
Score a patient across four criteria — degree of thrombocytopenia, timing of platelet fall, thrombosis or other sequelae, and alternative causes — to derive the 4Ts pre-test probability score for heparin-induced thrombocytopenia (HIT).
Thrombocytopenia — extent of platelet fall
Timing of platelet count fall
Thrombosis or other sequelae
Other causes for thrombocytopenia
Range 0–8: 0–3 low, 4–5 intermediate, 6–8 high HIT probability
- 1
Thrombocytopenia score (T1)
2 - 2
Timing score (T2)
2 - 3
Thrombosis score (T3)
0 - 4
Other causes score (T4)
2 - 5
4Ts total score
2 + 2 + 0 + 2 = 6Range 0–3 low, 4–5 intermediate, 6–8 high HIT probability.
How does this calculator work?
The 4Ts score sums four criteria (each 0–2): degree of thrombocytopenia, timing of platelet fall, thrombosis/sequelae, and alternative causes. Score 0–3 = low HIT probability (<1%); 4–5 = intermediate; 6–8 = high (>80%). Intermediate/high scores require laboratory confirmation and immediate heparin cessation. This is a clinical decision-support tool — not a stand-alone diagnosis.
Formula
How this is calculated
Heparin-induced thrombocytopenia (HIT) is an immune-mediated adverse drug reaction caused by antibodies against platelet factor 4–heparin complexes. Untreated, it carries significant thrombotic risk. The 4Ts score (Warkentin, 2008) is the most widely validated pre-test probability tool for HIT; a low score (<4) has a high negative predictive value and can significantly reduce unnecessary anticoagulant switches and laboratory testing.
Each of the four "T" criteria is scored 0, 1, or 2 based on clinical and laboratory findings. Thrombocytopenia is scored by the degree and nadir of the platelet fall. Timing is scored by when the fall began relative to heparin exposure — onset at days 5–10 (the classic immune window) scores highest; a fall in the first 4 days without prior heparin exposure scores 0. Thrombosis scores new proven thromboembolism, skin necrosis, or acute systemic reaction highest. Other causes scores inversely — a clear alternative explanation for thrombocytopenia reduces the likelihood of HIT and scores 0.
The total score stratifies risk: 0–3 is low probability (<1% HIT), 4–5 is intermediate (~10–30%), and 6–8 is high (>80%). For intermediate or high scores, confirm with a functional assay (SRA or HIPA) or immunoassay (anti-PF4/heparin ELISA). Stop heparin and switch to a non-heparin anticoagulant when HIT is suspected, pending confirmation. This calculator is a decision-support tool; clinical management must be directed by a qualified physician.
Frequently asked questions
A score of 0–3 is low probability for HIT (negative predictive value ~99%). Routine HIT antibody testing is generally not recommended, and heparin does not need to be stopped on the basis of the score alone. Clinical correlation with the full picture remains important.
A score of 6–8 indicates high probability HIT (>80%). Stop all heparin immediately (including flushes), initiate a non-heparin anticoagulant (e.g. argatroban, fondaparinux, bivalirudin), and confirm with a functional assay (SRA or HIPA). Do not switch to warfarin until the platelet count has recovered.
No. The 4Ts score estimates pre-test probability but does not confirm HIT. A low score can substantially reduce the likelihood and avoid unnecessary testing; an intermediate or high score requires confirmation with laboratory assays (immunoassay and/or functional assay). Clinical context and expert judgment are essential.
Also known as
TG we-Calculate Editorial Team. (2026). 4Ts Score Calculator — Heparin-Induced Thrombocytopenia (HIT) [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/4ts-score-calculator
TG we-Calculate Editorial Team. "4Ts Score Calculator — Heparin-Induced Thrombocytopenia (HIT)." TG we-Calculate. 2026. https://we-calculate.com/calculator/4ts-score-calculator.
TG we-Calculate Editorial Team, "4Ts Score Calculator — Heparin-Induced Thrombocytopenia (HIT)," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/4ts-score-calculator
@misc{wecalculate_4ts_score_calculator, title = {4Ts Score Calculator — Heparin-Induced Thrombocytopenia (HIT)}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/4ts-score-calculator}}, year = {2026}, note = {TG we-Calculate} }
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