Methods for quantitating extra-cellular vesicle surface markers, and compositions for practicing the same

A method of quantitating a surface marker on extra-cellular vesicles of a sample, the method comprising:

• comparing:
• (i) a mean fluorescence intensity of the surface marker (surface marker MFI) of a labeled extra-cellular vesicle (EV) sample, wherein the labeled EV sample has been labeled with a surface marker label comprising a specific binding member for the surface marker and a fluorophore;
• with
• (ii) a calibration plot obtained from a liposome calibration composition comprising two or more distinct liposome subpopulations each comprising a known amount of the fluorophore that differs from the amount of any other subpopulation in the liposome calibration composition, wherein:
• the fluorophore is covalently bound to the surface of a liposome or encapsulated within a liposome, and;
• the calibration plot comprises a plot of the known amount of the fluorophore for each of the distinct liposome subpopulations vs. a fluorescence intensity for each of the distinct liposome subpopulations;
• to obtain a count of the fluorophores bound to the surface of extracellular vesicles of the EV sample and quantify the surface marker on extra-cellular vesicles of the EV sample.