The invention provides a method and apparatus obtaining a hematocrit from a sample of in vivo tissue. The method comprises irradiating the sample with a single incident wavelength on a sample of tissue, simultaneously measuring wavelength shifted (IE) and unshifted (EE) light emitted from the tissue, and determining a relative volume of light emitted from two phases, wherein the two phases comprise a first Rayleigh and Mie scattering and fluorescent phase associated with red blood cells, and a second, non-scattering phase associated with plasma. The hematocrit is calculated from the volume of light emitted by the first phase relative to the total volume of light emitted from the first and second phases.
