Hi Andrew,
I don't know if one lab is more reliable than another. But I do know that the last set of numbers from LabCorp was suspicious since my M-spike was larger than my total IgG (as Terry pointed out). I therefore wanted to use a different lab for a reality check in case there was an equipment malfunction at LabCorp. However, I remember this same lab M-spike / IgG inconsistency once happening to me before with Quest.
I'm guessing this is a one-time event and I will probably go back to using LabCorp since I find their lab reports to be a bit more comprehensive. My oncologist's hospital lab farms a fair amount of their work out to Quest and also does some testing internally. My GP uses LabCorp and has a satellite office in his facility. But I just find it easier to go directly to an independent LabCorp or Quest lab in town and simply bypass the doctor's offices altogether. That way I get the results directly without having my doctor or the facility controlling their release to one of their portals or holding the reports ransom until my doctor discusses the lab results with me.
If my numbers look steady, I don't even bother going in to see my oncologist to discuss the results any more (and he's cool with that, given our relationship). But if there is some worrisome variance in my lab numbers or I haven't seen my oncologist in say 9-12 months, or if I'm due to get some imaging, then I go in and see him. This saves me a fair amount on office visit copays and works because I'm only smoldering and I'm not getting any treatment.
My oncologist's only advice to me regarding this practice was to stick with one lab since LabCorp and Quest have different reference ranges and use different equipment to run the tests. In this case, the Quest and third-to-last test results from LabCorp seem to generally agree.
This is an interesting article on LabCorp versus Quest. It also references the now infamous Theranos company. If you don't know the scandalous and lurid story about Theranos, you should. The story is now chronicled in the book "Bad Blood: Secrets and Lies in a Silicon Valley Startup", which will soon be turned into a movie.
https://www.mountsinai.org/about/newsroom/2016/mount-sinai-researchers-assess-accuracy-of-commercially-available-lab-tests