I just read a couple sites that suggested a folate deficiency could result in normal B12 levels but large red blood cells. If you had a folate deficiency, that would mean the serum test would show B12 in your system but would result in an inability to metabolize and absorb the B12. Better tests would be a MMA test and I think it's called a homocysteine test. If those two measures were elevated, that would confirm that you aren't absorbing the B12. Here's the link:
http://www.livestrong.com/article/44...ls-normal-b12/
'...Causes
A finding of large red blood cells and normal vitamin B-12 levels points to several possibilities. Alcoholism may create large red blood cells. Low folate levels and some drugs such as methotrexate, used to treat some cancers or rheumatic or immune conditions, interfere with folate metabolism and may cause the condition. Folate, another B vitamin, and vitamin B-12 are both involved in some of the same chemical reactions required to make normal sized red blood cells. Therefore, low levels of either of these B vitamins can lead to large red blood cells. In addition, low thyroid hormone levels, liver disease, bone marrow disorders and pregnancy are other causes of large red blood cells with normal B-12 levels.....'