How are exosomes counted?

Due to their size, the concentration of exosomes within a solution is typically assessed by protein assay, either Western blot or Bradford assay. Western blot analysis of the protein content in exosomes requires an antibody against one of the major proteins commonly found in exosomes, such as CD-63 or MHC Class II. As exosomes vary in size and composition, this value cannot be directly extrapolated to numbers of exosomes, but is adequate for most applications. For a direct assessment of exosome numbers, there are three possible methods: electron microscopy, nanoparticle tracking analysis, ELISA-based quantitative immunoassay, or a combination of these techniques.