4Ts score (heparin-induced thrombocytopenia)
Pre-test probability of HIT
- Validated in Greifswald and Hamilton
- Onkopedia Gerinnung (DGHO)
Sources last checked: 2026-10-13
4Ts score (heparin-induced thrombocytopenia)
Pre-test probability of HIT
The negative predictive value of a low score is high; the positive predictive value of intermediate and high scores varied widely between the two derivation centres. A score does not replace PF4/heparin immunoassay and functional testing.
Interpretation
| 0–3 — low probability | Clinically significant HIT antibodies in 0–1.6 % across the two derivation centres. ASH 2018 advises against HIT testing and empiric treatment at a low score. |
|---|---|
| 4–5 — intermediate probability | HIT antibodies in 7.9–28.6 % across the two centres. ASH 2018 provides for laboratory testing (PF4/heparin immunoassay) at an intermediate score. |
| 6–8 — high probability | HIT antibodies in 21.4–100 % across the two centres. ASH 2018 provides for laboratory testing at a high score and describes management as suspected HIT pending the result. |
What is the 4Ts score (heparin-induced thrombocytopenia)?
The 4Ts score estimates the pre-test probability of heparin-induced thrombocytopenia (HIT) from four features: the degree of thrombocytopenia, its timing, thrombosis or other sequelae, and the presence of other causes. Each is scored 0–2 for a total of 0–8. Lo and colleagues evaluated it prospectively in 2006 in Hamilton (Canada) and Greifswald (Germany), where a low score almost never coincided with HIT antibodies.
When to use it
- In any patient on, or recently exposed to, heparin whose platelet count falls
- Before ordering PF4/heparin antibody tests, to decide whether testing is needed
- To document clinical reasoning when stopping or continuing heparin
When not to use it
- As a confirmatory test — intermediate and high scores need laboratory confirmation
- When platelet counts are missing so that the timing or extent cannot be judged; testing may then be needed anyway
How it is calculated
| Criterion | Points |
|---|---|
| Thrombocytopenia | |
| Fall > 50 % and nadir ≥ 20 ×10⁹/L | 2 |
| Fall 30–50 % or nadir 10–19 ×10⁹/L | 1 |
| Fall < 30 % or nadir < 10 ×10⁹/L | 0 |
| Timing of platelet fall (heparin start = day 0) | |
| Clear onset day 5–10, or fall ≤ 1 day with heparin exposure in the past 30 days | 2 |
| Consistent with day 5–10 but unclear (e.g. missing counts), onset after day 10, or fall ≤ 1 day with heparin 30–100 days ago | 1 |
| Fall before day 4 without recent heparin exposure | 0 |
| Thrombosis or other sequelae | |
| New proven thrombosis, skin necrosis at injection sites, or acute systemic reaction after IV heparin bolus | 2 |
| Progressive or recurrent thrombosis, non-necrotising skin lesions, or suspected but unproven thrombosis | 1 |
| None | 0 |
| Other causes of thrombocytopenia | |
| None apparent | 2 |
| Possible | 1 |
| Definite | 0 |
Clinical pearls and limitations
- The negative predictive value of a low score is high; the positive predictive value of intermediate and high scores varied widely between the two derivation centres. A score does not replace PF4/heparin immunoassay and functional testing.
- Day 0 is the first day of the current heparin course; a rapid fall within 24 h suggests recent (≤ 30 days) prior exposure.
- The fall is measured from the highest count after heparin started, not from the normal range.
- 'Other causes' is where most disagreement lies — sepsis, DIC, drugs and post-operative haemodilution are common alternatives.
- A low score (0–3) has a high negative predictive value; ASH 2018 advises neither testing nor empirical treatment in that case.
Frequently asked questions
What 4Ts score rules out HIT?
A low score of 0–3 makes HIT unlikely; in the 2006 evaluation only 0–1.6 % of low-score patients had clinically significant HIT antibodies.
When does HIT typically occur?
Five to ten days after starting heparin, or within a day if the patient received heparin in the previous 30 days.
What does an intermediate 4Ts score mean?
HIT is possible but far from certain; ASH 2018 provides for a PF4/heparin immunoassay at this score. Whether to switch to a non-heparin anticoagulant while awaiting the result is addressed in detail in the guidelines.
Evidence and guidelines
- Primary source: Lo GK, Juhl D, Warkentin TE, et al. 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.
- Onkopedia (DGHO): Diagnostik von Gerinnungsstörungen — Tabelle 12, 4T-Score.
- Cuker A et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: heparin-induced thrombocytopenia. Blood Adv 2018;2(22):3360-3392.
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