A method and apparatus that allows accurate spectrophotometric determination of the concentrations of various hemoglobin species in whole undiluted blood. The invention employs 1) an optical apparatus designed to maximize the true optical absorbance of whole blood and to minimize the effects of light scattering on the spectrophotometric measurements of the concentrations of various constituent components, and 2) methods to correct the hemoglobin concentration measurements for light scattering and for the effects of the finite bandwidth of the substantially monochromatic light. In the optical apparatus optical parameters, such as sample thickness, detector size and shape, sample-to-detector distance, wavelengths, monochromicity, and maximum angle of light capture by detector, are selected so as to minimize the contribution of light scattering to maximize the contribution of true optical absorbance. After making measurements of a blood sample's optical density at each of the wavelengths, corrections are made for the effects of light scattering.