The present invention is directed to method and apparatus for non-invasively measuring the concentration of an unknown blood analyte present in a patient. The method involves determining the concentration of a reference analyte present in the patient. Using the non-invasive optical instrument of the invention, the concentrations of the reference analyte and unknown analyte are measured. If the measured reference analyte concentration and determined reference analyte concentration are substantially the same, then the measured unknown analyte concentration is displayed. If the measured reference analyte concentration and determined reference analyte concentration are not substantially the same, then an adjustment factor is applied to the measured unknown analyte concentration and the unknown analyte concentration is displayed.