Plasma and serum inhibit PF4 binding to platelets. (A) PRP, GFPs, or washed platelets were incubated with increasing concentrations of PF4. PF4 binding was detected with a FITC-labeled anti-human PF4 antibody using flow cytometry, and antibody binding was quantified by GMFI. Data are mean ± standard deviation (SD) of 6 independent experiments. (B) GFPs were incubated with 25 µg/mL PF4 in the presence of increasing concentrations of plasma or serum. PF4 binding was detected with an FITC-labeled anti-human PF4 antibody using flow cytometry, and antibody binding was quantified by GMFI. Surface-bound PF4 without addition of plasma or serum was defined as 100%. Data are mean ± SD of 4 independent experiments. *P < .05, **P < .01 vs PF4 without plasma or serum.