What is the 4T score?

By William Aird

A clinical prediction rule used to predict the likelihood (pretest probability) of HIT. Includes a consideration of magnitude of platelet count drop, timing of platelet count drop, presence of thrombosis and presence of alternative causes of the reduced platelet count. See calculator.

Parameter2 points1 point0 pointsComment
ThrombocytopeniaPlatelet count fall > 50% AND platelet nadir ≥ 20 × 109 L−1Platelet count fall 30%–50% OR platelet nadir 10–19 × 109 L−1Platelet count fall < 30% OR platelet nadir < 10 × 109 L−1Fall from highest platelet count that immediately precedes the putative HIT-related platelet count fall. 95% of cases of HIT are reported to develop in temporal association with heparin therapy; typically > 50% platelet count fall, but not to levels < 20 × 10 9 /L; only a few patients show 30%-50% platelet count fall; typical nadir is 40-80 × 109/L, with median of 55 × 109/L.
Timing of platelet count fall

Clear onset between days 5 and 10 OR platelet fall ≤ 1 day (prior heparin exposure within 30 days)Consistent with days 5–10 fall, but not clear (e.g. missing platelet counts) OR onset after day 10 OR fall ≤ 1 day (prior heparin exposure 30–100 days ago)Platelet count fall < 4 days without recent heparin exposureDay 5 to 10 for initial platelet count fall with day 0 representing first heparin exposure; earlier fall if patient exposed to heparin with previous 30 days. Days are rounded off. For example, day 4.3 would count as day 4.
Thrombosis or other sequelae

New thrombosis (confirmed) OR skin necrosis at heparin injection sites OR acute systemic reaction after intravenous heparin bolusProgressive or recurrent thrombosis or non-necrotizing (erythematous) skin lesions or suspected thrombosis (not proven.None
Other causes for thrombocytopeniaNone apparentPossibleDefinite

Scoring 0, 1, or 2 points for each of 4 categories, maximum possible score = 8 Low score (0-3 points), intermediate score (4 or 5 points), high score (6-8 points).

  • Low score 0-3 points
  • Intermediate score 4-5 points
  • High score 6-8 points

Low 4Ts score may rule out HIT but high 4Ts score may not be sufficient to diagnose HIT.