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Diagnostic Criteria for Heparin-Induced Thrombocytopenia (HIT)

  • 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)





>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)


OTher cause(s) of platelet fall

None evident



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.


Related Criteria

Classification of Neutropenia

Classification of Neutrophilia

Serum Levels That Differentiate Anemia of Chronic Disease from Iron-Deficiency Anemia

Diagnostic Criteria for Disseminated Intravascular Coagulation (DIC)

Clinical Conditions Associated with Disseminated Intravascular Coagulation (DIC)

Diagnostic Criteria for Idiopathic Hypereosinophilic Syndrome (HES)

Diagnostic Criteria for Systemic Mastocytosis (SM)

WHO Classification of Mastocytosis

FAB Classification of Myelodysplastic Syndromes (MDS)




  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]

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Created: September 22, 2005
Last Modified: 10/23/2010

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