I asked my neurologist why he only did samples from my lower leg and hip area. I have neuropathy in my arms and legs. He said that the normal values (all the studies that have been done) focused on samples from legs (since it is more common and is more often the first location for presenting symptoms of peripheral neuropathy). It's the same with blood pressure, and all other readings that are done. The "normal values" are drawn from conclusive evidenced based medicine/studies. So whatever populations they studied or locations they performed a test on, that's more "standardized" and interpretable.

Hope that helps!
-J