i will thanks

short answer, washout is to prevent most recent supplemental intakes from clouding background status.
long answer:
when i was silly pre dx, even so i had b12 on my radar due to diet and i monitored my levels. but i actually used to take b12 before my bloodwork. levels always came back low normal, but i knew from changes in symptoms that it didn't reflect my true status quo. out of curiousity i did a washout before testing once, and the b12 levels were undetectable - test was sensitive down to 75 pmol/l and the lab report said 'less than 75'.
iater when things got dire and the ms tests were done, i argued against the ms dx.
'no i'm b12 deficient'.
'but your test came back 300 pmol/l'.
'i've been supplementing like mad because of all this crazy new sensory loss'.
'so do a washout for us and retest'.
'no i'm terrified to stop now that things have gotten so much worse'.
and the rest is history.
there are other weird things to consider also, re bloodwork. eg coffee before testing can affect serum uric acid results, making them look higher than they really are.
also, morning fasting serum zinc is the highest level of the day so when i test that way, which i always do for comparability's sake, i have to be aware that it's my best possible result over a 24 hour period. makes my last serum zinc result, 12 umol/l, look even worse than just 'low normal' (it's actually great per the local lab but not per the hospital's system, which is more consistent with research)
with my zinc that low i can imagine my serum uric acid levels were crap too, but i don't have that on my next requisition. my old doc retired and i'm working in a new one. very slowly :S lol