For healthcare professionals
4Ts score calculator
The 4Ts score estimates the pretest probability of heparin-induced thrombocytopenia (HIT) in a patient on heparin whose platelet count falls. Four items score 0, 1 or 2 points each. Lo and colleagues published it in 2006. This calculator is written for clinicians and is not a diagnosis.
Patient findings
Pick one answer in each group. Nothing you enter leaves this page.
What you will see
The 4Ts score, the points behind it, and the pretest probability band.
Educational only, not medical advice. Not a diagnosis. Testing and treatment decisions belong to the treating team.
Source
Lo GK and colleagues, Journal of Thrombosis and Haemostasis 2006. The criteria and bands are taken from the open-access (CC BY) table by Larsen and colleagues, JAMA Network Open 2024, and the probabilities from Capuano and colleagues, Clinical Kidney Journal 2026 (CC BY). The tests check every band edge and the platelet count helper.
Validated in
Patients on heparin with suspected HIT. Larsen and colleagues studied 1318 consecutive patients with suspected HIT at 11 centers in Switzerland, Germany and the United States (median age 67; the most frequent setting was the intensive care unit).
Limits
In that study the 4Ts score produced 9.0% false negatives and 49.0% false positives against a functional platelet activation test (Larsen 2024). A low score does not rule out HIT in every setting: guidelines still advise testing in some groups, such as patients in intensive care (Capuano 2026).
Points
| Item | 2 points | 1 point | 0 points |
|---|---|---|---|
| Thrombocytopenia | Fall over 50% and nadir 20 × 10⁹/L or more | Fall of 30% to 50%, or nadir 10 to 19 × 10⁹/L | Platelet count fall under 30%, or nadir under 10 × 10⁹/L |
| Timing of the platelet count fall after heparin | Clear onset on days 5 to 10, or within 1 day with heparin in the past 30 days | Days 5 to 10 but onset unclear (for example, a missing count), after day 10, or within 1 day with heparin 30 to 100 days ago | 4 days or less, with no heparin in the past 100 days |
| Thrombosis or another sequela | New thrombosis, skin necrosis, or acute systemic reaction after an intravenous unfractionated heparin bolus | Progressive or recurrent thrombosis, non-necrotizing skin lesions, or suspected thrombosis | None |
| Other causes of thrombocytopenia | None | Possible | Definite |
Source: Larsen and colleagues 2024 (Larsen 2024), compiled from Lo and colleagues (Lo 2006). The total runs from 0 to 8.
Worked example: a platelet fall of 60% with a nadir of 40 × 10⁹/L (2), a clear onset on day 7 of heparin (2), no thrombosis (0) and another possible cause (1) give a 4Ts score of 5, intermediate probability.
What the score means
| Score | Pretest probability | Probability of HIT |
|---|---|---|
| 0 to 3 | Low | Under 5% |
| 4 to 5 | Intermediate | 14% |
| 6 to 8 | High | 64% |
Source: Capuano and colleagues 2026 (Capuano 2026). In that review, a low 4Ts score excludes HIT and laboratory testing is not recommended, with exceptions such as patients in the intensive care unit. With a score of 4 or more, guidelines recommend stopping heparin, starting an alternative anticoagulant and confirming the diagnosis with immunological and functional tests (Capuano 2026).
In the prospective study by Larsen and colleagues, 111 of 1318 patients (8.4%) had HIT on the reference test. The 4Ts score produced 9.0% false negatives and 49.0% false positives (Larsen 2024).
Related calculators
For suspected clots, see the Wells score for DVT and the Wells score for PE. For clot risk in medical inpatients, use the Padua prediction score. Other bedside scores are listed under For healthcare professionals.
Sources
- Larsen EL, Nilius H, Studt JD, et al. Accuracy of Diagnosing Heparin-Induced Thrombocytopenia. JAMA Netw Open. 2024;7(3):e243786. doi:10.1001/jamanetworkopen.2024.3786. PMID 38530310. Open access (CC BY). Table 1: 4Ts score criteria and bands (compiled from Lo 2006); 1318 patients with suspected HIT at 11 centers; 4Ts false negatives 9.0%, false positives 49.0%. pmc.ncbi.nlm.nih.gov/articles/PMC10966416/
- Capuano I, Riccio E, Buonanno P, Tufano A, Pisani A. Heparin-induced thrombocytopenia: a challenging diagnosis in haemodialysis, state of art and review of the literature. Clin Kidney J. 2026;19(4):sfag062. doi:10.1093/ckj/sfag062. PMID 42111239. Open access (CC BY 4.0). Table 1: probability of HIT under 5%, 14% and 64% by 4Ts band; testing and treatment by score. pmc.ncbi.nlm.nih.gov/articles/PMC13153468/
- Lo GK, Juhl D, Warkentin TE, Sigouin CS, Eichler P, Greinacher A. Evaluation of pretest clinical score (4 T's) for the diagnosis of heparin-induced thrombocytopenia in two clinical settings. J Thromb Haemost. 2006;4(4):759-765. doi:10.1111/j.1538-7836.2006.01787.x. PMID 16634744. Original paper (cited; the criteria used here are taken from Larsen 2024, CC BY). doi.org/10.1111/j.1538-7836.2006.01787.x
Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.