A variety of devices, systems, kits, and methods are provided for separating or enriching circulating tumor cells in a biological sample such as whole blood. In some aspects, the devices are multi-stage devices including at least a filtering stage, and two separation stages for ferrohydrodynamic separation of magnetically labelled white blood cells and for marker-independent and size-independent focusing of magnetically labeled particles so as to separate or enrich unlabeled rare cells in the biological sample. The devices and methods are, in some aspects, capable of high throughput in excess of 6 milliliters per hour while achieving high separation (>97%) of the unlabeled rare cells.