A method for determining the quantity of an analyte in a fluid is described along with various components of an apparatus designed to carry out the method. The method involves habituating a patient's eye to one or more colors, measuring the recovery time, and correlating the recovery time to the quantity of the analyte. A reproducible, objective, non-attentiveness-dependant test for assessing analyte levels is further disclosed. To this end, a device that measures saccadic eye movements may be used to assess the return of vision to the prehabituated state. The method and apparatus are particularly suited for noninvasively measuring blood glucose levels.