Measurement of PF4/heparin particle size and particle stability over time. (A) PCS measurements were taken of solutions containing mPF4 (200 μg/mL) before and after addition of various concentrations of UFH (0.5, 5, and 50 U/mL). Particle size was largest upon addition of 5 U/mL UFH (PHR, 2.6:1) and smallest with 50 U/mL UFH (PHR, 1:5). (B) Evolution of PF4/heparin particle size over time. UFH (0.5, 5, or 50 U/mL) was added to 200 μg/mL PF4 at time 0, and particle size was measured every 8 minutes over the ensuing 2 to 3 hours until particle size stabilized. Serial measurements of particle sizes are shown.