Lo et al reported the 4T score for the diagnosis of heparin-induced thrombocytopenia (HIT). The authors are from McMaster University in Canada and Ernst-Moritz-Arndt University in Germany.
platelet count fall > 50% AND platelet nadir >= 20 * 10^9/L
2
platelet count fall 30-50% OR platelet nadir 10-19 * 10^9/L
1
platelet count fall < 30% OR platelet nadir < 10 * 10^9/L
0
timing of fall in platelet count
clear onset between days 5-10 OR {platelet fall <= 1 day with prior heparin exposure within 30 days}
2
consistent with days 5-10 fall but not clear OR onset after day 10 OR {fall <= 1 day with prior heparin exposure 30-100 days ago}
1
platelet count fall < 4 days without recent exposure
0
thrombosis or other sequelae
new thrombosis; skin necrosis; acute systemic reaction after IV bolus of unfractionated heparin
2
progressive or recurrent thrombosis; non-necrotizing skin lesions; suspected thrombosis but not proven
1
none
0
other cause of thrombocytopenia
none apparent
2
possible
1
definite
0
where:
• The "nadir platelet count" is ambiguous. It could refer to the lowest platelet count, or the decrease from baseline to the lowest platelet count. The latter will be used in the implementation.
total score =
= SUM(points for all 4 parameters)
Interpretation:
• minimum score: 0
• maximum score: 8
Total Score
Probability of HIT
0 to 3
low
4 or 5
intermediate
6 to 8
high
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