4Ts score for heparin-induced thrombocytopenia

What goes into the 4Ts score

• Thrombocytopenia: Fall < 30% or nadir < 10 (0); Fall 30-50% or nadir 10-19 (1); Fall > 50% and nadir ≥ 20 (2)
• Timing of fall: Day ≤ 4 with no recent heparin (0); Day > 10, unclear, or ≤ 1 day with heparin 30-100 days ago (1); Day 5-10, or ≤ 1 day with heparin in past 30 days (2)
• Thrombosis: None (0); Progressive/recurrent or suspected thrombosis, non-necrotizing skin lesions (1); New confirmed thrombosis, skin necrosis, or systemic reaction to heparin bolus (2)
• Other cause of low platelets: Definite other cause (0); Possible other cause (1); None apparent (2)

How to interpret the result

• 0–3: low probability (negative predictive value about 99%); HIT testing usually not needed
• 4–5: intermediate; stop heparin, start a non-heparin anticoagulant and send a PF4 immunoassay
• 6–8: high; stop all heparin, start a non-heparin anticoagulant, send PF4 immunoassay and a functional assay

Clinical pearls

• Stop heparin flushes and heparin-coated lines too.
• Avoid platelet transfusion and do not start warfarin until platelets recover.

Evidence

1. Lo GK, 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;4:759-765.
2. Cuker A, et al. American Society of Hematology 2018 guidelines for management of venous thromboembolism: heparin-induced thrombocytopenia. Blood Adv. 2018;2:3360-3392.

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