PF4 and Thrombocytopenia: An EIA identifies anti-platelet factor 4/heparin IgG antibody (PF4 Ab) that activates platelets triggering serotonin release which is a functional assay and is highly specific for the diagnosis of HIT. Heparin-induced thrombocytopenia is considered unlikely for low clinical probability patients on the 4Ts scoring (Figure 1) [13].