The Make magazine blog points to an interesting hack of personal glucose meters by Yi Lu and Yu Xiang at the University of Illinois Urbana-Champaign: They figured out a way to use them to measure chemicals other than glucose.
Li and Xiang reasoned that, if they could find a way to chemically couple a compound with glucose, i.e. a reaction that would produce one molecule of glucose for each molecule of the target compound, then a PGM could be used just as well to measure the target compound. Then they went and found a way to do that for, well, just about any compound a person might want to measure.
The process requires some fancy chemistry to raise a DNA fragment that will bind specifically to the target molecule, but once that’s done, the reagent can be produced and sold in bulk inexpensively. You would buy a reagent custom-designed for your analyte of interest, mix it with your sample, add a pinch of sugar (literally), and the sugar would be converted to glucose in direct proportion to the concentration of your target. Then stick a grocery store PGM in the vial and take a reading.