• Heparin exposure >5 days
• Relative thrombocytopenia: decrease in platelet count by 50% from baseline OR absolute thrombocytopenia: decrease in platelet count to less than 100 to 150 x 10⁹/L
• Absence of other causes of thrombocytopenia
• Development of new thrombosis, or extension of pre-existing thrombosis, while receiving heparin therapy
• Confirmation by laboratory testing
• Return to normal platelet count when heparin is discontinued

Serology tests to determine HIT (listed in order of greatest to lowest sensitivity)

• Serotonin release assay
• Heparin/PF 4 ELISA
• Platelet aggregation

 

Estimating the Pretest Probability of HIT: The "Four T's"

	Points (0, 1, or 2 for Each of 4 Categories: Maximum Possible Score = 8)
	2	1	0
Thrombocytopenia	>50% Platelet fall to nadir >/= 20	30–50% Platelet fall, or nadir 10-19	<30% Platelet fall, or nadir <10
Timing* of onset of platelet fall (or other sequelae of HIT)	Days 5–10, or </= day 1 with recent heparin (past 30 days)	>Day 10 or timing unclear; or <day 1 with recent heparin (past 31–100 days)	<Day 4 (no recent heparin)
Thrombosis or other sequelae	Proven new thrombosis; skin necrosis; or acute systemic reaction after intravenous UFH bolus	Progressive or recurrent thrombosis; erythematous skin lesions; suspected thrombosis (not proven)	None
OTher cause(s) of platelet fall	None evident	Possible	Definite

UFH, Unfractionated heparin

Pretest probability score: 6–8 indicates high; 4–5, intermediate; and 0–3, low.

*First day of immunizing heparin exposure considered day 0.

 

References:

1. Warkentin TE, Aird WC, Rand JH. Platelet-endothelial interactions: sepsis, HIT, and antiphospholipid syndrome. Hematology (Am Soc Hematol Educ Program). 2003;:497-519. [Medline]
2. Warkentin TE, Cook DJ. Heparin, low molecular weight heparin, and heparin-induced thrombocytopenia in the ICU. Crit Care Clin. 2005 Jul;21(3):513-29. [Medline]