A method of identifying target sub-populations of nucleated cells contained in a quiescent sample of anticoagulated whole blood which is admixed with a fluorescent colorant, wherein the blood sample is diluted by no more than about 1:1, wherein said sample of anticoagulated whole blood is dispersed in a viewing chamber that includes a plurality of different thickness regions, including: a first region having a first through plane thickness between two and seven microns that permits inclusion of individual red blood cells and/or monolayers of red blood cells; and a second region having a second through plane thickness between seven and forty microns; said method comprising: a) the step of scanning selected fields of view (1,3,5) in said regions with an optical scanning instrument; b) the step of illuminating said selected fields of view (1,3,5) with a light source of one or more preselected wavelengths that are operable to cause said fluorescent colorant to produce fluorescent emissions at one or more known wavelengths, which emissions are characteristic of said target sub-populations of nucleated cells; and c) by virtue of the cells' characteristic fluorescence at said known wavelengths including 530nm, 585nm and 660nm when excited at the preselected wavelength in the range of 485nm, and by their fluorescence at said other known wavelength of 610nm when excited at the preselected wavelength in the range of 560nm, the step of analysing said emissions at said known wavelengths so as to identify and differentiate said target sub-populations of nucleated cells; wherein the chamber has opposite sample-containment walls (4,8), and the term "through plane" refers to a line of sight which corresponds to the shortest distance between the walls (4,8) in a given region of the chamber. a first region having a first through plane thickness between two and seven microns that permits inclusion of individual red blood cells and/or monolayers of red blood cells; and a second region having a second through plane thickness between seven and forty microns; said method comprising: a) the step of scanning selected fields of view (1,3,5) in said regions with an optical scanning instrument; b) the step of illuminating said selected fields of view (1,3,5) with a light source of one or more preselected wavelengths that are operable to cause said fluorescent colorant to produce fluorescent emissions at one or more known wavelengths, which emissions are characteristic of said target sub-populations of nucleated cells; and c) by virtue of the cells' characteristic fluorescence at said known wavelengths including 530nm, 585nm and 660nm when excited at the preselected wavelength in the range of 485nm, and by their fluorescence at said other known wavelength of 610nm when excited at the preselected wavelength in the range of 560nm, the step of analysing said emissions at said known wavelengths so as to identify and differentiate said target sub-populations of nucleated cells; a) the step of scanning selected fields of view (1,3,5) in said regions with an optical scanning instrument; b) the step of illuminating said selected fields of view (1,3,5) with a light source of one or more preselected wavelengths that are operable to cause said fluorescent colorant to produce fluorescent emissions at one or more known wavelengths, which emissions are characteristic of said target sub-populations of nucleated cells; and c) by virtue of the cells' characteristic fluorescence at said known wavelengths including 530nm, 585nm and 660nm when excited at the preselected wavelength in the range of 485nm, and by their fluorescence at said other known wavelength of 610nm when excited at the preselected wavelength in the range of 560nm, the step of analysing said emissions at said known wavelengths so as to identify and differentiate said target sub-populations of nucleated cells; wherein the chamber has opposite sample-containment walls (4,8), and the term "through plane" refers to a line of sight which corresponds to the shortest distance between the walls (4,8) in a given region of the chamber.