Clinical probability score for Heparin-Induced Thrombocytopenia.
Clinical Scoring
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.
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.
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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.
