An apparatus for separation and analysis of a target analyte in a sample of anticoagulated whole blood, comprising: a flexible sample tube for holding the sample, the sample tube having an elongate sidewall of a first cross-sectional inner diameter, which tube is at least semi-transparent, or optionally transparent said sidewall being resiliently radially expandable to a second diameter in response to centrifugal force, said second diameter being sufficiently large to permit axial movement of a float in the tube during centrifugation; an elongate, rigid, volume-occupying float (110, 210, 310, 410, 510, 610, 710, 810, 910, 1010, 1110, 1210, 1310, 1410, 1610, 1710, 1810, 1910, 2010. 2110, 2210, 2310, 2A 10, 2510, 2610, 2710, 2810) having a specific gravity intermediate that of red blood cells and plasma, said float comprising: a main body portion and one or more support members protruding from the main body portion to engage and support the sidewall of the sample tube, said main body portion and said support members having a cross-sectional diameter less than said second inner diameter of the tube when the sample tube is expanded, wherein said main body portion together with an axially aligned portion of said sidewall define an annular volume therebetween; and wherein said support members traverse said annular volume to produce one or more analysis areas; and characterized in that the annular volume between said main body portion and the axially aligned portion of said tube sidewall has a radial thickness of from 25 μm to 250 μm when the tube is not expanded, wherein the thickness is sized by the height of the support members. a flexible sample tube for holding the sample, the sample tube having an elongate sidewall of a first cross-sectional inner diameter, which tube is at least semi-transparent, or optionally transparent said sidewall being resiliently radially expandable to a second diameter in response to centrifugal force, said second diameter being sufficiently large to permit axial movement of a float in the tube during centrifugation; an elongate, rigid, volume-occupying float (110, 210, 310, 410, 510, 610, 710, 810, 910, 1010, 1110, 1210, 1310, 1410, 1610, 1710, 1810, 1910, 2010. 2110, 2210, 2310, 2A 10, 2510, 2610, 2710, 2810) having a specific gravity intermediate that of red blood cells and plasma, said float comprising: a main body portion and one or more support members protruding from the main body portion to engage and support the sidewall of the sample tube, said main body portion and said support members having a cross-sectional diameter less than said second inner diameter of the tube when the sample tube is expanded, wherein said main body portion together with an axially aligned portion of said sidewall define an annular volume therebetween; and wherein said support members traverse said annular volume to produce one or more analysis areas; and characterized in that the annular volume between said main body portion and the axially aligned portion of said tube sidewall has a radial thickness of from 25 μm to 250 μm when the tube is not expanded, wherein the thickness is sized by the height of the support members. a main body portion and one or more support members protruding from the main body portion to engage and support the sidewall of the sample tube, said main body portion and said support members having a cross-sectional diameter less than said second inner diameter of the tube when the sample tube is expanded, wherein said main body portion together with an axially aligned portion of said sidewall define an annular volume therebetween; and wherein said support members traverse said annular volume to produce one or more analysis areas; and characterized in that the annular volume between said main body portion and the axially aligned portion of said tube sidewall has a radial thickness of from 25 μm to 250 μm when the tube is not expanded, wherein the thickness is sized by the height of the support members.