1. A method for assessing the need for hemodialysis, the progress of a hemodialysis procedure or the adequacy of a hemodialysis treatment, said method comprising the steps of: providing a means for optical analysis of tissue on a patient, said means providing an output spectrum at multiple wavelengths, wherein said output spectrum has varying intensities as related to absorption by said tissue; coupling said tissue to said means for optical analysis and acquiring said output spectrum before, during or after hemodialysis; and, calculating a tissue urea concentration from a mathematical model relating said output spectrum to said tissue urea concentration.