I think testing is a waste here, as the standard tests measure what is in the blood and it is it what is inside the cell that matters.

Also, what impact would a measurement have? I think the test subjects in the thiamine Vietnamese American study from 2012 all had thiamine levels in the "normal" range, but they all had PD and all felt horrible and all benefitted immensely from thiamine supplementation.

"Normal" is relative as well. Maybe I need 25mg/day of some nutrient to function optimally, someone else may need 100mg/day.