An optical system and method for quantifying total protein in whole blood or other multi-phase liquids and colloidal suspensions uses refractometry without preliminary steps such as cell separation or centrifugation. A refractometer is integrated with a flow cell to enable the refractive index of a flowing sample to be measured based on a substantially cell free boundary layer of the sample that is present under certain flow conditions. Dimensions of the flow cell are selected to produce a cell-free layer in a flow of whole blood in which the cell free layer is thick enough to reduce scattering of light from the refractometer light source. A numerical method is used to compensate for scattering artifacts. The numerical compensation method is based on the slope and width of a peak in the derivative curve of an angular spectrum image of the flowing sample produced by refractometry.