Improving the scatter sensitivity of a flow cytometer for detection of extracellular vesicles

Extracellular vesicles (EVs) are commonly studied by flow cytometry. Due to their small size and low refractive index, the scatter intensity of most EVs is below the detection limit of common flow cytometers. University of Amsterdam researchers set out to improve forward scatter (FSC) and side scatter (SSC) sensitivity of a common flow cytometer to detect single 100 nm EVs.

The effects of the optical and fluidics configuration on scatter sensitivity of a FACSCanto (Becton Dickinson) were evaluated by the separation index (SI) and robust coefficient of variation (rCV) of polystyrene beads (BioCytex). Improvement is defined as increased SI and/or reduced rCV. Changing the obscuration bar improved the rCV 1.9-fold for FSC. A 10-fold increase in laser power improved the SI 19-fold for FSC and 4.4-fold for SSC, whereas the rCV worsened 0.8-fold and improved 1.5-fold, respectively. Confocalization worsened the SI 1.2-fold for FSC, and improved the SI 5.1-fold for SSC, while the rCV improved 1.1-fold and worsened 1.5-fold, respectively. Replacing the FSC photodiode with a photomultiplier tube improved the SI 66-fold and rCV 4.2-fold. A 2-fold reduction in sample stream width improved both SI and rCV for FSC by 1.8-fold, and for SSC by 1.3- and 2.2-fold, respectively. Decreasing the sample flow velocity worsened rCVs. Decreasing the flow channel dimensions and the pore size of the sheath filter did not substantially change the SI or rCV. Using the optimal optical configuration and fluidics settings, the SI improved 3.8∙104 -fold on FSC and 30-fold on SSC, resulting in estimated detection limits for EVs (assuming a refractive index of 1.40) of 246 and 91 nm on FSC and SSC, respectively.

Although a 50-fold improvement on FSC is still necessary, these adaptions have produced an operator-friendly, high-throughput flow cytometer with a high sensitivity on both SSC and FSC.

(A) Schematic representation of the adaptations made to increase sensitivity.  (B) Side scatter (SSC‐H) histogram of beads (blue) and noise (red).