PF4 Immunoassays in Vaccine-Induced Thrombotic Thrombocytopenia

. All tests were performed with or without unfractionated heparin at 0.1 IU per milliliter (UFH 0.1), 0.5 IU per milliliter (UFH 0.5), and 10 IU per milliliter (UFH 10). Clinically significant and strong platelet activation, with maximum release ranging from 36 to 99%, was measured in seven of the nine patients only when PF4 was present in the reaction mixture. Moreover, platelet activation was not inhibited by therapeutic concentrations of UFH 0.1 or UFH 0.5. In contrast, platelet activation was completely abolished by 10 μg per milliliter of IV.3, a monoclonal antibody specific for FcγRIIA, or 6 U of IdeS, an IgG-degrading enzyme of Streptococcus pyogenes , preincubated for 15 minutes at 37°C in the plasma sample of each patient tested (five patients) before SRA