A more elaborate configuration of the apparatus with a salt bridge, made of 2% agar in incubation buffer, allowed conductive contact with the nitrocellulose near the spot of glucose oxidase and the buffer in the upper reservoir and increased the linear range. This salt bridge was easily damaged by drying, however, when the system was temporarily unused. Moreover, for diabetes, most informative urinary glucose concentrations lie in the linear range of the method we present here. Accurate quantitative information for higher concen- trations is usually not needed in practice, and these concentrations are labeled simply as “very high” (in the event that it is useful to measure higher concentrations, an extra dilution step could be added to enable better quantitation). We note that, for monitoring diabetes, low concentrations in urine (in the range of 0−2 mM) are sometimes not informative,
because they can be caused by mixing urine with a small volume of urine with a high glucose concentration already in the bladder (that had remained in the bladder after emptying). In buffer, we estimated the lower limit of detection to be about 0.2 mM. This lower limit was enabled by a lower background in buffer relative to urine.