4Ts Score for HIT

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Clinical probability score for Heparin-Induced Thrombocytopenia.

Clinical Scoring

Thrombocytopenia

Timing of Platelet Count Fall

Thrombosis or Other Sequelae

Other Causes for Thrombocytopenia

HIT Probability

Total Score0/ 8

ProbabilityLow

Probability of HIT is low (<1%). Consider other causes.

Diagnostic Recommendation

Probability Scale

0 - 3Low Probability

4 - 5Intermediate

6 - 8High Probability

If HIT is strongly suspected (Intermediate/High), all heparin including flushes must be discontinued immediately.

About this calculator

Overview

The 4Ts score is a pre-test clinical probability tool used to assess the likelihood that a patient has Heparin-Induced Thrombocytopenia (HIT). It helps clinicians decide whether to stop heparin and start alternative anticoagulation while waiting for laboratory confirmation.

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Pro Tips

Base timing on the first day the patient received heparin during the current admission.
A 'Nadir' is the lowest platelet count reached during the episode.
Wait for functional assays (e.g., SRA) to confirm diagnosis, but treat based on clinical suspicion if the score is intermediate or high.

Fun Facts

"A low score (0-3) has a negative predictive value of ~99% for ruling out HIT."
"Intermediate and high scores require cessation of all heparin and specialist consultation."
"HIT is a paradox: low platelets combined with a high risk of life-threatening thrombosis."
"Recent heparin exposure (within 30 days) allows for 'Rapid Onset' HIT within hours."

Formula: Total Score = Thrombocytopenia + Timing + Thrombosis + Other Causes.