Visual Quality Control With <scp> <i>CytoMDS</i> </scp> , a Bioconductor Package for Low Dimensional Representation of Cytometry Sample Distances

Quality Control (QC) of samples is an essential preliminary step in cytometry data analysis. Notably, the identification of potential batch effects and outlying samples is paramount to avoid mistaking these effects for true biological signals in downstream analyses. However, this task can prove to be delicate and tedious, especially for datasets with dozens of samples. Here, we present CytoMDS, a Bioconductor package implementing a dedicated method for low-dimensional representation of cytometry samples composed of marker expressions for up to millions of single cells. This method allows a global representation of all samples of a study, with one single point per sample, in such a way that projected distances can be visually interpreted. CytoMDS uses Earth Mover's Distance for assessing dissimilarities between multi-dimensional distributions of marker expression and Multi-Dimensional Scaling for low-dimensional projection of distances. Some additional visualization tools, both for projection quality diagnosis and for user interpretation of the projection coordinates, are also provided in the package. We demonstrate the strengths and advantages of CytoMDS for QC of cytometry data on three real biological datasets, revealing the presence of low-quality samples, batch effects, and biological signal between sample groups.