Light microscopy is listed in the pharmacopeias (Ph. Eur. 2.9.19 and USP <788>) as alternative compendial method to light obscuration for the analysis of subvisible particles. The sample solution is filtered onto a membrane and particles larger than 10 µm and larger than 25 µm are counted manually. The advantage of light microscopy is a direct visualization of the sample.
However, filtration can influence sample properties and methods such as the compendial light obscuration method are considered clearly more quantitative and operator-independent. In order to overcome the inability of light obscuration to visualize the sample we suggest flow imaging microscopy techniques such as Micro-Flow Imaging which can be fully validated under GMP at Coriolis.