The more the better – determining the optimal range when performing single-vesicle phenotyping

The characterization of extracellular vesicles (EVs) has evolved rapidly in recent years due to advances in straightforward technologies. Based on these more sensitive methods, it is now possible to describe EV populations in their entirety more precisely. However, these applications require an equivalently delicate experiment design and optimization steps to draw valid conclusions in the end. One of these methods is represented by the highly sensitive nanoflow cytometry (nFCM), by which particles can be analyzed not only on their size (< 40 nm) and concentration but also concerning surface markers. In this work, we addressed some of the potential caveats of this method, especially when characterizing particles with fluorescently labelled antibodies. In particular, we show, when using low particle concentrations, which are inevitably encountered when working with EVs, the characterization of surface markers is prone to significantly varying. We hypothesized that these technical limitations could respond to the stickiness of EVs and should be properly counteracted. As a reference, we strongly recommend performing particle number-based comparisons with at least 109 particles as staining input in nFCM analyses. Moreover, we provided representative particle-number based immunoblotting results, underlying the significance of this parameter as a normalizer in future EV research.