HIT is an immune reaction to heparin that paradoxically causes clots. A low 4Ts score (0–3) has a negative predictive value above 99%, so HIT testing and stopping heparin are usually not needed; intermediate and high scores need laboratory testing and a non-heparin anticoagulant.
How to use the 4Ts Score Calculator
- Thrombocytopenia
- Timing of platelet fall
- Thrombosis or other sequelae
- Other causes of thrombocytopenia
Results update as you type, and every field accepts the units shown in its menu.
How it works
Each answer carries the points shown in the published score. The calculator adds them up and maps the total to the published interpretation.
Scoring table
| Item | Answers and points |
|---|---|
| Thrombocytopenia | Platelet fall > 50% and nadir ≥ 20 ×10⁹/L = 2; Fall 30–50% or nadir 10–19 = 1; Fall < 30% or nadir < 10 = 0 |
| Timing of platelet fall | Days 5–10, or ≤ 1 day with heparin in the past 30 days = 2; Consistent with days 5–10 but unclear, after day 10, or ≤ 1 day with heparin 30–100 days ago = 1; Before day 4 without recent heparin = 0 |
| Thrombosis or other sequelae | New thrombosis, skin necrosis or acute systemic reaction = 2; Progressive, recurrent or suspected thrombosis; non-necrotising skin lesions = 1; None = 0 |
| Other causes of thrombocytopenia | None apparent = 2; Possible = 1; Definite = 0 |
Inputs explained
| Field | What to enter |
|---|---|
| Thrombocytopenia | Choose one: Platelet fall > 50% and nadir ≥ 20 ×10⁹/L, Fall 30–50% or nadir 10–19, Fall < 30% or nadir < 10 |
| Timing of platelet fall | Choose one: Days 5–10, or ≤ 1 day with heparin in the past 30 days, Consistent with days 5–10 but unclear, after day 10, or ≤ 1 day with heparin 30–100 days ago, Before day 4 without recent heparin |
| Thrombosis or other sequelae | Choose one: New thrombosis, skin necrosis or acute systemic reaction, Progressive, recurrent or suspected thrombosis; non-necrotising skin lesions, None |
| Other causes of thrombocytopenia | Choose one: None apparent, Possible, Definite |
Health note: this calculator is for education and screening only and is not a diagnosis or treatment advice. Discuss your results with a qualified clinician.
Formula
Score = sum of the points for each item
Worked example
Inputs — Thrombocytopenia: Platelet fall > 50% and nadir ≥ 20 ×10⁹/L; Timing of platelet fall: Days 5–10, or ≤ 1 day with heparin in the past 30 days; Thrombosis or other sequelae: None; Other causes of thrombocytopenia: Possible.
Results — 4Ts score = 5 / 8; Probability of HIT = Intermediate (about 10%).
Key terms
- HIT
- Heparin-induced thrombocytopenia.
Frequently asked questions
What should be done with a low score?
HIT is very unlikely; testing is usually not needed.
What if the score is intermediate or high?
Stop all heparin, start a non-heparin anticoagulant and send HIT antibody tests.
Does HIT occur with low-molecular-weight heparin?
Yes, but less often than with unfractionated heparin.