1. A method for non-invasive quantitative detection of a physiological chemical in a test subject comprising the steps of: (a) irradiating an exterior surface of a portion of the test subject with near-infrared radiation such that the radiation is transmitted through or reflected from the portion of the test subject; (b) collecting data concerning the infrared radiation transmitted through or reflected from the portion of this subject using a detector, wherein the collected data is from an absorbance spectrum or an interferogram; (c) digitally filtering the collected data to isolate a portion of the data indicative of the physiological chemical; and (d) determining the amount of physiological chemical in the test subject by applying a defined mathematical mode to the digitally filtered data.