Table 3. Establishing the Pre-Test Probability for Suspected Heparin-induced Thrombocytopenia: The 4T's.

The 4T's	2 Points	1 Point	0 Points
Thrombocytopenia	> 50% fall or nadir of 20–100 × 109 cells/L	30%–50% fall or nadir 10–19 × 109 cells/L	< 30% fall or nadir < 10 × 109 cells/L
Timing of thrombocytopenia (relative to heparin)	5–10 days or < 1 day of heparin reexposure within 30 days	Beyond day 10 or unclear	< 5 days no reexposure
Thrombosis	Proven (skin necrosis, thrombosis)	Recurrent or suspected	None
eTiology of thrombocytopenia	No other cause evident	Possible	Definite

Scoring: 0–3, low probability; 4—5, moderate probability; 6–8, high probability.

Adapted from Curr Hematai Rep.⁵⁶