Standardization of extracellular vesicle concentration measurements by flow cytometry: the past, present, and future

Concentrations of extracellular vesicles (EVs) in body fluids are being explored as disease biomarkers. Most laboratories use flow cytometry to characterize single EVs at high throughput. A flow cytometer (FCM) detects light scattering and fluorescence intensities of EVs. However, detection of EVs by flow cytometry is complicated for 2 reasons. First, EVs are small and have weak light scattering and fluorescence signals compared to cells and are, therefore, hard to detect. Second, FCMs differ in sensitivity and provide data in arbitrary units, which complicates data interpretation. Due to the mentioned challenges, the measured concentration of EVs by flow cytometry is cumbersome to compare between FCMs and institutes. To improve comparability, standardization and development of traceable reference materials to calibrate all aspects of an FCM are needed, as are interlaboratory comparison studies. Within this article, we will provide an overview of the standardization of EV concentration measurements, including the current effort to introduce robust calibration of FCMs, thereby enabling comparable concentration measurements of EVs, which in turn can be used to establish clinically relevant reference ranges of EV concentrations in blood plasma and other body fluids.