--I think anything up to about 500-550 is suspect--at least if one is eating a diet with animal protein sources in it (vegetarians/vegans are very often affected by lowering B12 levels as they do not get it from diet and we cannot manufacture it ourselves; such people should definitely supplement).

There have been people who have been symptomatic of B12 deficiency even into the 600 range.

I suspect that the original normal ranges for B12 are set too low at that end. Japanese labs, due to the drug-induced B12 crisis that Mrs. D has talked about here, tend to set the low end of their lab ranges around 500-550, and many have no upper limit. (I also think the upper limit of some labs in the 900 range are a little ridiculous--if one absorbs B12 well on a non-vegetarian diet, or if one supplements, especially with a daily B-complex, one's readings are likely to be higher. My wife takes a daily B-complex with 125mcg of B12 in it and her levels are usually in the 1100's. I take methylcobalamin 1000mcg three times/week and get readings in the 1500's-1800's. This, of course, frightens uniformed doctors unless you explain it to them--if they're willing to listen.)