I have access to a spectrometer and run samples for my friends (for free). Iron has ranged from 5 ppm to a high of 29 ppm on one engine that is flown regularly. Without knowing what the engine typically produces, how would the owner have known that the 29 wasn't a problem? On an engine that has flown irregularly--not much in the winter--the high has been 94 ppm. These high numbers show up every spring, because he doesn't fly much in the winter. There is probably some rust going on, but where would you draw the line for a tear down? The next sample, in early summer typically drops down to the 20's. Without knowing the pattern of use and what is typical for a particular engine, you have little useful information for a single sample. And with that high of 94, the proper action was to resample later after flying more.
Sampling on a regular basis is the only reliable method for the wear metals. Water, pH, etc. may be another story, but we don't measure those at the lab I have access to.