4 Ts score for estimating the pretest probability of heparin-induced thrombocytopenia (HIT)

4 Ts score parameters:
Thrombocytopenia:
PLT decrease >50% AND nadir ≥20,000/microL AND no surgery within preceding 3 days	2 points
PLT decrease >50% BUT surgery within preceding 3 days OR any combination of PLT fall and nadir that does not fit criteria for 2 or 0 points (eg, 30 to 50% fall or nadir 10,000 to 19,000/microL)	1 point
PLT decrease <30% OR nadir <10,000/microL	0 points
Timing of onset after heparin exposure:
5 to 10 days OR 1 day if exposure within past 5 to 30 days	2 points
Probable 5 to 10 days (eg, missing PLT counts) OR >10 days OR <1 day if exposure within past 31 to 100 days	1 point
≤4 days without exposure within past 100 days	0 points
Thrombosis or other clinical sequelae:
Confirmed new thrombosis, skin necrosis, anaphylactoid reaction, or adrenal hemorrhage	2 points
Suspected, progressive, or recurrent thrombosis, skin erythema	1 point
None	0 points
Other cause for thrombocytopenia:
None	2 points
Possible (eg, sepsis)	1 point
Probable (eg, DIC, medication, within 72 hours of surgery)	0 points
Interpretation:
0 to 3 points – Low probability (<1%)
4 to 5 points – Intermediate probability (approximately 10%)
6 to 8 points – High probability (approximately 50%)

HIT is a clinical and laboratory diagnosis, and this score is not intended to take the place of clinical judgment by a clinician with experience in diagnosing and managing HIT. Refer to UpToDate for details of the evaluation.

PLT: platelet; DIC: disseminated intravascular coagulation.

Adapted from: 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; 4:759.

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4 Ts score for estimating the pretest probability of heparin-induced thrombocytopenia (HIT)