An apparatus is presented for estimating the concentration of an analyte using a combined enzyme-spectroscopic method. Examples are provided for the detection of glucose and lactate. A sample of biological fluid is mixed or contacted with an enzyme specific to the analyte of interest, and the reacting fluid is probed with an optical system at wavelengths that includes at least one wavelength that is sensitive to the analyte concentration and at least one wavelength that is not sensitive to the analyte concentration. The optical system measures properties, such as optical density, and relates the measurements to concentration through a calibration of the system. A method is also provided for analyzing the data obtained from optical measurements of reactions of enzymes with biological fluids. These technologies may be applied to continuous or periodic patient sampling systems or to test strip type devices.