No vitamins before test?

I have tried to ask this a few days ago but it got lost in the posts; so apologies to those who have already seen it (I got no replies)

If lab test are supposed to show us what we need (increase, decrease intake etc.:

Why fast from vitamins if we are supposed to take them daily until the day we die and the point of taking them is EXACTLY to keep the in-range levels in our system?

So what is the point of testing what the levels might be without them in the system? by fasting ?  Do we need to know what we would be like without? Or with?

I would really appreciate some logical/scientific reply if someone is willing. THANKS!
Also I have not  heard from the visits that I am supposed to do this - maybe precisely because what actually needs to be tested is the way we metabolize vitamins on a daily basis and with daily intake of the regimen?

 If you take vitamins within 24 hours of labwork, the lab results will show the level of current vitamins in your blood, NOT what your actual body reserves are.  So, with the high levels of vitamins we take, the results will appear "within range" because we are still metabolizing the recent intake.  If you withhold the recent intake, you get the complete picture of what you have to USE, not just what you took an hour ago that is still processing.  It is imperative as DSers to see what our STORES are, and if any of those are low, how low they are, so we can adjust our intake to raise our stores.  Labs are NOT to see what our body is doing an hour after we take our dose of vites, they are to check on what our body has in reserve.

ETA- I'd get more technical, but I have a raging migraine and can't see straight.  Literally.  

 I was not given those instructions.  Just fasting after midnight the night before labs the next morning was all I was told.  The DSrs I know do this also.  

There are always 2 schools of thought on this when we discuss it, but I like bean's post.   Here's an old post   http://www.obesityhelp.com/forums/DS/3432606/Vitamin-A-Blund ers/action,replies/topic_id,3432606/page,1/

I used to stop vits for 2 days, now I just do one.

Thank you all !
I will read what you sent.

 I'm a pre op but in the past I was told by the lab techs that drinking water is ok and that taking your meds are ok as long as you dont have to take them with food.