The present invention provides a method for identifying a concentration of a compound within a part of a subject. The method involves measuring the amount of electromagnetic radiation reflected by, or transmitted through the part with a detector, and using a quantitative mathematical analysis to determine the concentration of the compound in one, or more than one compartment, including a blood, interstitial, cellular, lymph, or bone compartment, of the part. A corrected concentration of the compound may be then determined within a compartment of interest, or a total concentration of the compound may be accurately determined. Preferably, the compound is glucose. From this determination, a clinical condition in a human or animal may be made by correlating the concentration of a measured compound in the compartment of a part of the human or animal to a clinical condition in need of treatment.