The 4Ts Score is a clinical prediction tool developed to assess the pretest probability of heparin-induced thrombocytopenia, a rare but serious immune-mediated adverse effect of heparin therapy. HIT occurs when antibodies form against platelet factor 4 (PF4)–heparin complexes, leading to platelet activation, thrombocytopenia, and a markedly increased risk of thrombosis. Because thrombosis rather than bleeding is the major complication, early recognition is critical.

The 4Ts mnemonic reflects the four key clinical domains:

1. Thrombocytopenia – degree of platelet count fall.

2. Timing – when the fall occurs relative to heparin exposure.

3. Thrombosis – presence of new clot or skin necrosis.

4. oTher causes – possible alternative explanations.

Each domain is scored from 0 to 2, resulting in a total from 0 to 8. Patients are then classified as low, intermediate, or high probability for HIT. This score is primarily used as an initial screening tool. A low score has a high negative predictive value and can safely exclude HIT without further testing, while intermediate or high scores warrant laboratory confirmation such as PF4 ELISA or serotonin release assay.