A method for monitoring blood oxygen saturation (SO.sub.2) includes the steps of calculating an estimate of SO.sub.2 in the blood and correcting the estimate by a scaling factor. A preferred embodiment of the method includes the steps of transmitting light containing a plurality of wavelengths into the blood; measuring a remitted spectrum over the plurality of wavelengths; calculating a measured blood absorption spectrum from the remitted spectrum; estimating local rates of change in the measured blood absorption spectrum at a plurality of the wavelengths; and calculating the estimate of SO.sub.2 as a function of absolute values of the local rates of change of the measured blood absorption spectrum.