This thesis consists of two parts. Part I describes the development of a fully callibrated Imaging Flow Cytometry (IFCM)-based methodology for the direct detection of single Extracellular Vesicles (EVs) in complex, clinically relevant biofluids such as human plasma, urine, or perfusion fluids. Part II presents the application of the developped methodology in the clinical context of kidney transplantation (KTx) by demonstrating the characterization of different (kidney-derived) EV subsets released either before KTx (during Normothermic Machine Perfusion), or after KTx (direct-detection of donor-derived EVs in the blood plasma of kidney transplant recipients).