Heparin-induced thrombocytopenia diagnostic criteria
Pre-test clinical score for HIT/HITT
1. Sum the points for each factor
| Factor | 0 points | 1 point | 2 points |
|---|---|---|---|
| Thrombocytopenia | Platelet 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 exposure | Platelet count fall < 4 days without prior exposure in past 100 days | Consistent 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 sequelae | None | Progressive 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 thrombocytopenia | Definite | Possible | None |
2. Use total score to determine pre-test probability of detecting HIT antibodies
| Pretest probability | Score | Probability of detecting HIT antibodies (single-centre study) | Probability of detecting HIT antibodies (multi-centre study) |
|---|---|---|---|
| High | 6 - 8 | 100% | 21% |
| Intermediate | 4 - 5 | 29% | 8% |
| Low | ≤ 3 | 2% | 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.