Those pens and kits can be tricky, and part of the problem I THINK is, if the chemicals get old or are stored at high temps, the results can be off.
Also, I couldn't ever get a straight answer on whether they can detect the difference between real silver and silver plate, real gold and gold plate.
I actually only used them when I was buying and selling antique and vintage jewelry. Now that I make my own, I rely on the integrity of my suppliers. (Cue a round of applause for R&T here....)