The ‘4 T’s’ (thrombocytopenia, timing of platelet count fall, thrombosis or other sequelae, and other causes for thrombocytopenia,) enable the calculation of a pretest clinical score for the diagnosis of heparin-induced thrombocytopenia. In one series, a low pretest clinical score for HIT was associated with a high-negative predictive value. Although a low score might be suitable for ruling out HIT in some clinical settings, this approach has not yet been validated prospectively.

References
The ‘4 T’s’ score was developed and tested in two clinical settings (Hamilton, Canada and Greifswald, Germany) by 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 Haem 4(4):759-765.
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