Please let me explain how blood testing works.

First off the B12 blood level, may be misleading. It only measures what is floating around in the blood. If the B12 is not being consumed in the various tissues because it is not active, then it will appear normal or even high in the blood.

The MMA test, however, shows if B12 is actually WORKING.
This compound requires active B12 to remove it from our systems as it becomes toxic when high.
http://labtestsonline.org/understand...s/mma/tab/test

So if the MMA comes out high in testing, that shows that something is wrong with B12 in that person.

Another test to show if B12 is working, is homocysteine. This compound elevates when B12, folate, and activated B6 are not present in adequate amounts. These 3 vitamins are cofactors to methylate homocysteine to SAMe. The methyl groups are added to homocysteine , which becomes SAMe (s-adenosyl methionine) which then carries that methyl group to the liver, joints and nervous system where the various neurotransmitters are manufactured. So if homocysteine is elevated, then one needs to look at these cofactors.

The methylation chemistry is rather new. And the DNA testing also within the last decade or so. And the methylcobalamin is relatively new too. All within the last 10 yrs.

Why spend money on the older forms of these critical vitamins, when the activated ones, are available for about the same cost?

The basic screening for low B12 using serum testing, is only one tool. People may test above 400pg/ml today, and still need the supplements.
Here is an important link to Dr. C. Snow's article which explains this:
http://neurotalk.psychcentral.com/post698522-70.html

Other countries are way ahead of us about this subject.
Japan and India have had serious issues with B12 deficiency, and one can see that they treat B12 deficiency more aggressively than in the US. However, our labs in US still report low values as "normal"... that is below the new recommended low of 400pg/ml. And doctors continue to tell patients who are low that they are "normal", and the damage and suffering continue.

edit to add: There is a new test, called the transcobalamin test.
This is not yet available in all labs, and is expensive. But it is supposed to be the "best". I haven't done it yet, myself tho.
http://www.questdiagnostics.com/test...28&labCode=SJC

A hint of developing B12 problems can be seen in a CBC test which is
pretty routine at most doctors. (the tests above have to be requested specially).
This is the MCV or mean corpuscular volume. This test will elevate or hover around the upper range when B12 is becoming scarce. Often doctors completely ignore this result, and that is a shame because it
can point to a need for further testing. It measures the size of the cells in a given volume of blood and when B12 is not working the red cells are defective and larger than usual.