Investigation of Anti-PF4 versus Anti-PF4/Heparin Reactivities by Fluid-Phase EIA for Four Anti-PF4 Disorders: Classic HIT, Autoimmune HIT, VITT, Spontaneous HIT.

BACKGROUND: Four platelet-activating anti-PF4 disorders are recognized: classic heparin-induced thrombocytopenia (cHIT), autoimmune HIT (aHIT), spontaneous HIT (SpHIT), and vaccine-induced immune thrombotic thrombocytopenia (VITT); all test IgG-positive by solid-phase enzyme-immunoassay (solid-EIA) against PF4/heparin (PF4/H) and/or PF4 alone. A fluid-phase EIA (fluid-EIA) should better discriminate between anti-PF4 and anti-PF4/H antibodies since conformationally-altered PF4 bound to solid-phase is avoided.

OBJECTIVES: To compare anti-PF4 versus anti-PF4/H antibody profiles for anti-PF4 disorders using solid- and fluid-EIAs.

METHODS: We developed a novel fluid-EIA to measure anti-PF4 versus anti-PF4/H antibodies.

RESULTS: By fluid-EIA, 27/27 (100%) cHIT sera tested IgG-positive with PF4/H but only 4/27 (14.8%) tested positive against PF4 alone; all 27 exhibited heparin-enhanced binding. In contrast, 17/17 (100%) VITT sera tested IgG-positive against PF4 alone, with markedly reduced binding against PF4/H; this distinct VITT antibody profile was not evident by solid-EIA. All 15 aHIT sera and all 11 SpHIT sera tested IgG-positive against PF4 alone, with variable reactivity by PF4/H-EIA (heparin enhanced binding in 14/15 and 10/11 aHIT and SpHIT sera, respectively). Remarkably, one SpHIT patient with a VITT-mimicking fluid-EIA profile (PF4>>PF4/H) also resembled VITT clinically (post-viral cerebral vein/sinus thrombosis), with anti-PF4 reactivity correlating inversely with platelet count recovery.

CONCLUSIONS: CHIT and VITT sera showed opposite fluid-EIA profiles (cHIT: PF4/H>>PF4, with most testing negative against PF4 alone; VITT: PF4>>PF4/H, with most testing negative against PF4/H). In contrast, all aHIT/SpHIT sera reacted against PF4 alone, but with variable (usually enhanced) reactivity against PF4/H. VITT-mimicking clinical/serological profiles occurred in only a minority of SpHIT and aHIT patients.