The only valid test I now request for B12 is the MMA test, methylmalonic acid. Plain old blood B12 is next to - useless. I have seen patients with values of blood 1000 pmol/L show signs of B12 deficiency. Yes, true. One needs to be very careful of blood tests, it is easy to get sidetracked into believing that the values you get from the patient, if they fall in the "refernce ranges" are the actual values the patient requires for optimal health. Conventional values are determined on "recommended" daily amounts, not the "optimal" values, and you will discover like I have after years of seeing patients that we all have varying requirements for vitamins and minerals.