Criteria
4Ts score
The 4Ts clinical prediction tool can be used to estimate the probability of a patient having HIT. Points are scored (0, 1, or 2) for each of the 4 categories (maximum possible score = 8).[36][37]
Thrombocytopenia
2 points if >50% fall in platelet count to a platelet count nadir of ≥20 × 10⁹/L (≥20 × 10³/microlitre)
1 point if 30% to 50% fall in platelet count, or if the nadir is 10-19 × 10⁹/L (10-19 × 10³/microlitre)
0 points if <30% fall in the platelet count, or if the nadir is <10 × 10⁹/L (<10 × 10³/microlitre).
Timing* of onset of platelet fall (or other sequelae of HIT)
2 points if onset is 5-10 days after starting heparin, or <1 day if there has been recent heparin (within past 30 days)
1 point if onset is >10 days after starting heparin or if timing unclear; or if <1 day after starting heparin with recent heparin (past 31-100 days)
0 points if onset is within 4 days of first-time heparin exposure (no recent heparin).
Thrombosis or other sequelae
2 points if there is a proven new thrombosis, or heparin skin necrosis, or acute systemic reaction after intravenous unfractionated heparin bolus
1 point if there is progressive or recurrent thrombosis, or erythematous skin lesions, or suspected thrombosis (not proven)
0 points if no thrombosis or other finding.
oTher cause(s) of platelet fall
2 points if none evident
1 point if there is another possible cause
0 points if there is another definite cause
Pre-test probability score
High = 6-8 points (approximately 50% probability of HIT)
Intermediate = 4-5 points (approximately 10% probability of HIT)
Low = 0-3 points (<1% probability of HIT).
*First day of immunising heparin exposure is considered day 0. It is also worth noting that, if the patient undergoes surgery, the day that heparin is restarted after the procedure is considered day 0 of heparin exposure, even if the patient received heparin preoperatively. Surgery is a strong immunising risk factor for HIT and can, therefore, potentially reset the clock for the development of HIT. Similarly, if heparin is given intra-operatively, the surgery date becomes day 0.
A typical high score would be a patient who experiences a 50% drop in platelet count with a nadir ≥20 × 10⁹/L (>20 × 10³/microlitre) between days 5 and 10 of heparin exposure, and is found to have new thrombosis and no alternative explanation for the drop in platelet count.
The 4Ts score has been subject to greater evaluation than other tools, and is recommended by the American Society of Hematology.[36][37][38][39][40]
HIT expert probability (HEP) score[41]
Points from -3 to +3 are given for 8 categories: magnitude of fall in platelet count, timing of fall in platelet count, nadir platelet count, thrombosis, skin necrosis, acute systemic reaction, bleeding, other causes of thrombocytopenia.
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