Abstract: The absence of a defined optical pathlength for in vivo measurements creates problems for the noninvasive measurement of analyte concentration. These problems can be reduced by combining measurements made at several wavelengths and using the fact that normal renal function causes the concentration of water in whole blood to be tightly controlled. Hence, the concentration of water in arterial blood can serve as a useful internal standard for such measurements. The measurements are then procured so as to remove the dependency of concentration on path length traversed by the illuminating radiation and on the scattering properties of the volume through which the illuminating radiation propagates. Using this method, one can create improved calibration for measurements of absorbing constituents in arterial blood and thereby provide absolute concentration measurements of constituents such as hemoglobin and glucose in arterial blood.

Abstract: The invention relates to the enhancement of the signal-to-background ratio of a non-invasive measurement of the concentration of a blood constituent at a measurement site by applying an external pressure at a location near the measurement site. In one embodiment, sufficient pressure is applied proximate to a measurement site to stop blood flow. The pressure is then suddenly relased, thereby generating a blood bolus passing through the site. By illuminating the measurement site before and during the passage of the blood bolus and observing the interaction of the input radiation with the measurement site, the concentration of a blood constituent can be measured. In another embodiment, the venous pulse is occluded by applying a pressure midway between systolic and diastolic pressure. By illuminating the measurement site in the absence of a venous pulse, the signal-to-background ration can be enhanced and the concentration of a blood constituent can be measured.