4Ts — suspected HIT (ASH 2018)

4Ts — suspected HIT (ASH 2018)

Principle

Estimate clinical pretest probability of suspected immune HIT in an adult. Review platelet counts, exposure and competing causes before selecting each rating.

Scope

Four explicit clinical ratings using the ASH 2018 table, days 5–14. No platelet-fall, date or count calculation from the Case.

Interpretation limits

ASH 2018 operational table, days 5–14. Four manual clinical judgements, without automatic calculations from dates or platelet counts. An unknown item is not zero. Categories describe pretest probability, not a diagnosis. This pretest estimate does not by itself confirm or exclude HIT. Interpret it with the clinical context and appropriate investigations. Reassess when the information or clinical condition changes. This tool does not direct stopping or restarting heparin; it calculates no individual percentage.

Formula and logic

4Ts = thrombocytopenia + timing + thrombosis/sequelae + other cause (0 to 2 points each).

Interpretation

0–3

Low clinical pretest probability

4–5

Intermediate clinical pretest probability

6–8

High clinical pretest probability

Definition version : 1.1.0

Sources

• American Society of Hematology — Guide de poche HIT, table des 4T

  2018 · Page 2 table: domains, 0–2 ratings and days 5–14 window.

• Larsen et al. — Diagnostic Accuracy of the 4Ts Score for Heparin-Induced Thrombocytopenia

  2024 · Validation cohort using days 5–10; it informs performance limits without changing the selected ASH variant.