A method of segregating a first population of cells selected from circulating tumor cells (CTCs) and circulating endothelial cells (CECs) from blood cells in a fluid sample, wherein said method employs a device comprising: a body and a cover that define a void therebetween, the void containing a separation element that segregates an inlet region and an outlet region of the void, the separation element defining, together with a surface of the void, a channel that fluidly connects the inlet and outlet regions by way of a separating portion, the channel having an overall width at the separating portion and a height defined by the distance between the separation element and the surface of the void, at least one of the body, the cover, and the separation element bearing a segregating step disposed within and having a leading edge extending completely across the separating portion of the channel, whereby the channel is divided into an upstream portion on the inlet side of the leading edge and a lamellar downstream portion on the outlet side of the leading edge, the upstream portion of the channel being also lamellar in a region between the inlet region and the separation element; the height of the upstream portion being sufficient to facilitate passage therethrough of both said first population of cells and said blood cells, the height of the downstream portion being sufficiently large to facilitate passage therethrough of said blood cells and sufficiently small to inhibit passage therethrough of said first population of cells, and the length of the leading edge being greater than the overall width of the channel at the separating portion; and said method comprises urging said fluid sample through said channel whereby said first population of cells are segregated from said blood cells based on their inability to traverse said segregating step and optionally recovering said first population of cells that are unable to pass to the outlet region. a body and a cover that define a void therebetween, the void containing a separation element that segregates an inlet region and an outlet region of the void, the separation element defining, together with a surface of the void, a channel that fluidly connects the inlet and outlet regions by way of a separating portion, the channel having an overall width at the separating portion and a height defined by the distance between the separation element and the surface of the void, a separation element that segregates an inlet region and an outlet region of the void, the separation element defining, together with a surface of the void, a channel that fluidly connects the inlet and outlet regions by way of a separating portion, the channel having an overall width at the separating portion and a height defined by the distance between the separation element and the surface of the void, at least one of the body, the cover, and the separation element bearing a segregating step disposed within and having a leading edge extending completely across the separating portion of the channel, whereby the channel is divided into an upstream portion on the inlet side of the leading edge and a lamellar downstream portion on the outlet side of the leading edge, the upstream portion of the channel being also lamellar in a region between the inlet region and the separation element; the height of the upstream portion being sufficient to facilitate passage therethrough of both said first population of cells and said blood cells, the height of the downstream portion being sufficiently large to facilitate passage therethrough of said blood cells and sufficiently small to inhibit passage therethrough of said first population of cells, and the length of the leading edge being greater than the overall width of the channel at the separating portion;