Formation of PF4-heparin complexes. SEC-HPLC of PF4-heparin complexes formed at varying PF4-heparin ratios (PHRs). In the absence of heparin, PF4 migrates as a single peak at 23 minutes. In the presence of heparin, 2 populations of complexes are observed: ULCs and smaller complexes (SCs). A portion of the PF4 does not form complexes with heparin and elutes at 23 minutes. Data are expressed as the percentage of total counts of each experiment and represent the mean ± SEM of 3 independent experiments. (A) PF4; (B) PHR1.7:1; (C) PHR1.1:1; (D) PHR0.8:1; (E) PHR0.07:1; (F) PHR0.06:1.