Investigation of anti-PF4 versus anti-PF4/heparin reactivity using fluid-phase enzyme immunoassay for 4 anti-PF4 disorders: classic heparin-induced thrombocytopenia (HIT), autoimmune HIT, vaccine-induced immune thrombotic thrombocytopenia, and spontaneous HIT

•Fluid-phase enzyme immunoassay (EIA)—vs solid-phase EIA—avoids conformational alteration of platelet factor 4 (PF4).•Using fluid-phase EIA, classic heparin-induced thrombocytopenia (HIT) sera recognize PF4/heparin (PF4/H) but usually not PF4 alone.•In contrast, autoimmune HIT, spontaneous heparin-induced thrombocytopenia (SpHIT), and vaccine-induced immune thrombotic thrombocytopenia (VITT) sera bind to PF4 alone, with variable reactivity against PF4/H.•Most SpHIT sera reacted with PF4/H more than with PF4; however, 1 mimicked VITT clinically and using fluid-phase EIA (PF4 >> PF4/H).