Home / The HIT Patient / Estimating the Probability

Estimating the probability

Establishing a diagnosis of HIT can be challenging, particularly in patients with complicated medical conditions, as thrombocytopenia is a relatively common finding.  Since HIT is a life threatening condition it is important to consider the possibility of HIT when thrombocytopenia is present in patients treated with heparin.

The clinical features of HIT that help distinguish it from other forms of thrombocytopenia include the timing of onset, the magnitude of the decrease in platelet count, and the presence of thrombosis or other sequelae.

Laboratory tests to confirm a diagnosis of HIT are not always available in a timely fashion. Therefore, a scoring system, The 4T's, using the clinical criteria for HIT has been developed. The evaluation of this scoring system in Canada and Germany suggest that low scores are associated with a very low risk of clinically significant HIT, whereas a high score indicates a moderate or high risk of HIT (see next page) (1).

 1. Lo GK, Juhl D, Warkentin TE et al. Evaluation of pretest clinical score (4T's) for the diagnosis of heparin–induced thrombocytopenia in two clinical settings. J Thromb Haemost 2006;4:759-65.

The 4Ts