Apparatus (20, 100) for assaying a target
analyte in a localized tissue region (22) that may include the target and other analytes comprising: a
light source (34, 104) that illuminates the region with light at each of a plurality of wavelengths at which light is absorbed and / or scattered by tissue in the region wherein light at a least one of the wavelengths is absorbed or scattered by the target
analyte; a
signal generator (40) that generates signals responsive to intensity of the light from the
light source (34, 104) at different locations in the localized region (22); and a controller (32, 102) that: receives the generated signals; processes the signals to determine an
extinction coefficient for light in the localized region at each
wavelength; and determines the concentration of the target
analyte responsive to a solution of a set of
simultaneous equations having as unknown variables concentrations of a plurality of analytes in the region (22), one of which is the target analyte, wherein each equation in the set expresses a relationship between the
extinction coefficient, the absorption coefficient and / or the reduced
scattering coefficient for light at a different one of the plurality of wavelengths and at least one of the equations expresses a relationship between the
extinction coefficient and the reduced
scattering coefficient.