I think the MMA blood test is a good indicator.

I attended a conference last fall where the speaker said
she was trying to get MMA instead of serum B12 done for all
residents entering long term care in her state.

CSF levels of B12 are low in MS and Fibro. But I haven't yet seen
that they are diagostic for everyone. Lumbar puncture is a very
invasive test, with side effects and potential for infection.
Not really necessary for most people IMO. (but does make MONEY for the neuro who does it).

more here:
http://www.labtestsonline.org/unders...ma/sample.html

B 12 and MMA
 

I have had the MMA test which was okay in my case. No, I would never suggest a CSF test, I was only saying that it was stated on one of the reference sites that it was the only "perfect" measurement. I have FMS as well. It seems like the more I read here, the less likely it seems that changing the doseage form is sensible. If one takes enough by mouth of a methyl form, it should work. Also, I have homocysteine levels done which have been fine and are allegedly an early sign of deficiency of B 12.
Thanks again for all those who took the time to respond.
Karen S.