Quantitation of protein particles in parenteral solutions using micro-flow imaging. 
The U.S. and European Pharmacopeias require subvisible (> or =10 and > or =25 microm) and visible particulate testing of therapeutics to ensure their safety and suitability for clinical use.  The objective of this article is to compare the sizing and counting accuracies of light obscuration, which is the standard technique used to measure subvisible particulate matter, and Micro-Flow Imaging (MFI), a new imaging-based technology.  An immunoconjugate was selected as the model protein for this study since it could be induced to form particulate matter in PBS.  Light obscuration was performed as described in USP chapter <788> while MFI measurements were conducted per the manufacturer's procedures.  The two techniques yielded similar results when polystyrene standards were analyzed.  However, the MFI measurements indicated the presence of significantly more particles in the protein-containing solution compared to the light obscuration measurements.  The presence of nonspherical protein particles as well as particles that possess a refractive index similar to the solvent that they are in appear to be detected by MFI, but not by light obscuration, leading to the difference in the results.  Imaging-based technologies could aid in developing formulations and processes that would minimize the formation of protein particulates.