Haematology
4Ts Score for HIT
Probability of heparin-induced thrombocytopenia.
Education and reference only. Not a substitute for clinical judgement, local policy or product labelling. Always verify before clinical use. Values are calculated in your browser and never stored.
Background
The 4Ts score estimates the pre-test probability of heparin-induced thrombocytopenia (HIT), an immune reaction to heparin that paradoxically causes thrombosis. It assesses four domains, each scored 0–2: Thrombocytopenia (the magnitude of the platelet fall and its nadir), Timing of the fall relative to heparin exposure, Thrombosis or other sequelae, and the likelihood of oTher causes of thrombocytopenia. The total ranges from 0 to 8 and was designed to identify which patients need laboratory confirmation. It helps avoid both missed HIT and unnecessary changes of anticoagulant.
Interpreting the result
A score of 0–3 indicates low probability, with a high negative predictive value that makes HIT very unlikely and usually allows heparin to continue while seeking other causes. A score of 4–5 is intermediate and 6–8 is high probability; both warrant stopping heparin, starting an alternative non-heparin anticoagulant and sending confirmatory tests. Laboratory testing (an immunoassay, ideally confirmed by a functional assay) is needed because the score alone does not diagnose HIT. The score's main strength is reliably ruling HIT out when low.
Worked example
A patient on heparin for six days has a 60% platelet fall to a nadir above 20 (2), an onset on day six (2), a new deep-vein thrombosis (2) and no other obvious cause (2): total 8, a high-probability score. Heparin should be stopped, a non-heparin anticoagulant started and confirmatory HIT testing sent.
Pearls / pitfalls
- The greatest value of the 4Ts is its high negative predictive value: a low score reliably excludes HIT, but a high score is far less specific and still needs laboratory confirmation.
- Classic HIT timing is a platelet fall 5–10 days after starting heparin, or sooner with recent heparin exposure in the previous month.
- Inter-observer agreement is only moderate, especially when judging whether another cause of thrombocytopenia is present — apply it carefully.
- Do not wait for laboratory results before acting on a high score: stop heparin and start a non-heparin alternative, as untreated HIT is highly thrombotic.
Evidence & validation
Developed by Warkentin and validated by Lo et al. (2006) and a subsequent meta-analysis showing a high negative predictive value for low scores; it is endorsed in guidance from the American Society of Hematology and British Society for Haematology for HIT assessment.
Frequently asked questions
What does a low 4Ts score mean?
A score of 0–3 indicates low probability and reliably makes HIT very unlikely, owing to the score's high negative predictive value. Heparin can usually continue while other causes of the low platelets are investigated.
Does a high score confirm HIT?
No. A high score raises probability but is not specific, so laboratory confirmation with an immunoassay and ideally a functional assay is still required. Treatment changes should begin while awaiting results.
When does HIT typically occur?
The platelet count classically falls 5–10 days after starting heparin, or within a day if the patient had heparin in the previous month. Falls before day four without recent exposure are unlikely to be HIT.
What should I do with an intermediate or high score?
Stop all heparin, start a non-heparin anticoagulant and send confirmatory HIT tests. Untreated HIT carries a high risk of thrombosis, so do not simply stop heparin without alternative anticoagulation.
How reliable is the score between assessors?
Agreement is moderate, particularly when deciding whether another cause of thrombocytopenia exists. Careful, consistent application and laboratory confirmation help mitigate this.
References
- Lo GK, Juhl D, Warkentin TE, et al. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia. 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.
About the creator
Lo GK, Warkentin TE et al.
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