PF4 and heparin form visible aggregates in a heparin-dependent manner. Solutions containing mPF4 (10-200 μg/mL) were incubated with increasing concentrations of UFH (0-50 U/mL, shown on a log x-axis scale). Optical absorbance was measured at 280_nm (y-axis). Encircled UFH concentrations (0.5, 5, and 50 U/mL) and mPF4 at a concentration 200 μg/mL were chosen for further study in subsequent experiments. The inset shows changes in absorbance as a function of PF4 concentration.