Relationship of turbidity to the stages of platelet aggregation.

The process of platelet aggregation as detected by turbidity changes in the platelet aggregometer was studied relative to light scattering by large particles. For latex beads a plot of light scattering intensity/unit mass versus particle size gave increased light scattering intensity for small particle sizes but decreased scattering at large particle size. This behavior is predicted by Rayleigh-Gans theory. These results were related to the platelet aggregometer, an optical instrument used to measure the association of small particles (monomeric platelets) to large particles (platelet aggregates). Formalin-fixed platelets do not show changes in light transmission due to energy-requiring processes, such as shape change, so that turbidity changes in the presence of aggregating agents could be attributed to a change in platelet aggregation state. Small platelet aggregates showed increased turbidity compared to a similar mass of monomeric platelets. In fact, very large platelet aggregates that were visible to the unaided eye were needed to produce a decrease in light scattering intensity. Thus, turbidity can either increase or decrease with platelet aggregation depending on the size of the aggregates. Studies of platelet aggregation that show no initial increase in turbidity must be characterized by dominance of large platelet aggregates and monomeric platelets.