The level in the 600's is above the cut off that is used in other countries for B12 supplements. It is considered higher in US, but then we are very behind in this subject. Many doctors will still tell people in the 200's that they are "fine" or "normal".

If rose were here, this is what she'd say: (I have read her for about 10 yrs, and have seen this question often).

"One person's 600 is not the same as another's."

There is no way to know if the number is going up or coming down, unless you have testing done every 3 to 6 months.
One person may have no symptoms at 600... another may have many symptoms at 600.

When people show up HERE, with puzzling symptoms that defy logic, we have to assume that that person is unique in some way and responding this way. So perhaps the person coming here has a genetic need for higher blood levels of the substance being measured. Dr. Bruce Ames believes that people all vary in their genetic use of the B-complex family. And that eventually genetic testing will be accurate enough to illuminate their specific needs.

So while a number in the 600 range looks good, it is only telling us that it is not extremely low. It doesn't tell us what range YOU need to feel better.