We present an analysis of the microfluidic Dean migration of 2.5 µm particles, which do not meet focus criterion, in tall and low aspect ratio microchannels. We demonstrate the use of such low aspect ratio and tall spirals (h > 50 µm) for isolating high concentration (> 106 particles or cells/mL) micron sized particles without an initial off-chip dilution step. We specifically show the need for a sheath fluid for isolation and systematically analyze the particle stream profile (i.e. thickness and distance from the channel wall) as a function of downstream channel length and curvature ratio, with changes in the fluid velocity and the flow rate ratio of particles to sheath fluid (FRR). We also show that the width of the particle stream can control the particle migration and that a threshold stream width and Dean drag is necessary to initiate the particle stream migration from the channel wall. We then propose a design guide based on the selection of optimum curvatures, flow velocities and the FRRs required for achieving a narrow particle stream through a particular outlet. Finally, we use the design guide to demonstrate the isolation of bacteria from bladder epithelial cells. © 2021, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.