The limit of detection (LOD) is the lowest detectable concentration of the analyte using a particular analytical method. For the example given the LOD is in the micromolar range. Logically, the determined glucose concentrations are above the LOD (mM range).
You can estimate the LOD of a particular analytical method using a calibration curve/dilution row. Just plot signal intensities (AUCs) against increasing analyte concentrations. Then, insert the smallest detectable signal intensity (e.g. a peak with signal/noise=3) into the equation of the linear regression. You will obtain a concentration that corresponds to the LOD for your analyte of interest with your particular method.