I find the Doctor's Data testing to be reliable given that I have done it several times after supplementing (without Doctor's Data knowing about the supplements) only to find out that those particular mineral levels were a little too high from too much supplementation at times. When I dropped the supplements completely, the levels dropped back to normal or low during the next test 6 months later.
I doubt it is coincidence that her mercury level dropped dramatically (90%) after stopping her daily canned tuna habit. Whether the cans are steel or aluminum I don't know for sure since I have not purchased it myself and run a magnet over it.
Doctor's Data hair testing has identified toxicity from hair/makeup products in my girlfriend, which all dropped after she changed to more natural makeup. Mercury levels in my mother. Supplement levels in myself. And perhaps, though I'm not certain of it yet, levels of toxic metals in the ocean off San Diego since my brother is a surfer there and has a particularly strong toxic metal profile (including uranium). When both my girlfriend and I used Head & Shoulders for a while our zinc numbers went through the roof. When we tried Selsun Blue, our selenium numbers went way up. I have seen far too much to believe it is simply coincidence.
I am not here to say that all hair testing is bulletproof, but ICP-MS is a robust method for distinguishing metals at minute levels. I work for the market leader in ICP-MS equipment so I am familiar with it and our customers who utilize it for diagnostic and other testing. Not all hair testing is created equal though. External contamination, washing procedures, etc can cause major issues. I've only ever used Doctor's Data so I cannot speak to the other labs and their accuracy. Hair testing for me is a very cheap and effective option that compliments my other current favorite testing procedures (Labcorp, Spectracell).