A method for predicting blood glucose level of a person includes performing on the person a near-infrared spectral scan of a body part at a first group of wavelengths and at a second group of wavelengths, determining a first group of near-infrared absorbance values for the first group of wavelengths and a second group of near-infrared absorbance values for the second group of wavelengths, determining a first difference for the first group of near-infrared absorbance values and a second difference for the second group of near-infrared absorbance values, and calculating a blood glucose level for the person using the first and second differences.