Heparin-induced thrombocytopenia diagnostic criteria

Pre-test clinical score for HIT/HITT

1. Sum the points for each factor

Factor0 points1 point2 points
Thrombo­cyto­peniaPlatelet count fall < 30%, OR
platelet nadir < 10 x109/L
Platelet count fall 30-50%, OR
platelet nadir 10-19x109/L
Platelet count fall > 50%, AND
platelet nadir ≥ 20 x109/L
Timing of platelet count fall post-heparin exposurePlatelet count fall < 4 days without prior ex­posure in past 100 daysCon­sistent with day 5-10 fall, but data in­complete (e.g. missing platelet count), OR
onset after day 10, OR
fall ≤ 1 day and prior heparin ex­posure 30-100 days ago
Clear onset bet­ween days 5-10, OR
fall ≤ 1 day with prior hep­arin ex­posure < 30 days ago
Thrombo­sis or other sequelaeNonePro­gressive or re­current thrombo­sis, OR
Non-necro­tizing (erythem­atous) skin lesions, OR
Sus­pected thrombo­sis (not proven)
New thrombo­sis (con­firmed), OR
skin necrosis, OR
acute sys­temic reaction post-intra­venous un­fraction­ated heparin bolus
Other cause for thrombo­cyto­peniaDefinitePossibleNone

2. Use total score to determine pre-test probability of detecting HIT antibodies

Pretest probabilityScoreProbability of detecting HIT antibodies (single-centre study)Probability of detecting HIT antibodies (multi-centre study)
High6 - 8100%21%
Intermediate4 - 529%8%
Low≤ 32%0%

This pretest clinical score for heparin-induced thrombocytopenia (HIT; also known as heparin-induced thrombocytopenia with thrombosis, or HITT) was published in 2006 (1). It was prospectively applied in 100 patients at a single centre (Hamilton General Hospital, Canada), and in 236 patients across multiple centres in Germany and Austria. In both the single-centre and multi-centre cohorts, a low-risk clinical score had a high negative predictive value for the detection of HIT antibodies. The positive predictive value was markedly different in the two cohorts, thought to reflect differences in the application of the scoring system, and in the confirmatory HIT tests used.