A method of determining a measure of a tissue state (e.g., glycation end-product or disease state) in an individual. A portion of the tissue of the individual is illuminated with excitation light, then light emitted by the tissue due to fluorescence of a chemical with the tissue responsive to the excitation light is detected. The detected light can be combined with a model relating fluorescence with a measure of tissue state to determine a tissue state. The invention can comprise measuring the fluorescence lifetime in either time-domain or frequency domain modes. The invention can also comprise a variety of models relating fluorescence to a measure of tissue state, including a variety of methods for generating such models. For example, multivariate models can be developed that relate lifetime trends of one or more constituents to increasing propensity to diabetes and pre-diabetes. Other biologic information can be used in combination with the fluorescence properties to aid in the determination of a measure of tissue state. The invention also comprises apparatuses suitable for carrying out the method, including appropriate light sources, detectors, and models (for example, implemented on computers) used to relate detected fluorescence and a measure of tissue state.