Serotonin release assay (SRA) = gold standard
Anti-PF4 plus SRA has combined sensitivity of 99% 
Positivity determined by optical density (OD) reported with assay (same concept as a titer)
OD <1 = <5% chance of HIT
OD 1.4 = 50% chance of HIT
OD >2 = 90% chance of HIT
OD <1 = <5% chance of HIT
OD 1.4 = 50% chance of HIT
OD >2 = 90% chance of HIT
Heparin-Induced Thrombocytopenia 
Marrow infiltration 
Disseminated intravascular coagulation
Viral infections 
Liver disease induced coagulopathy
Marrow suppression 
Vitamin K deficiency
Congenital thrombocytopenia