It stands to reason that if one supplements with B12 on a daily basis, one should have quite high serum B12 levels, as the body is being flooded with B12--but that's not an indication if one was deficient before starting that regimen, nor is it a good indication if it is being utilized on the cellular level.

This is why a low serum B12 level has more specificity than a high one does--a low level is almost definitely an indicator that someting is awry, while a high may or may not be.

People with "OK" B12 serum levels who have not started taking supplements are advised (by us at least, if not many doctors) to have the MMA and homocysteine tests to check for the metabolites of B12 utilization--these are better tests of B12 status (though even they are not perfectly sensitive and specific).

But, anyone who is suspected of having a gastrointestinal condition that involves malabsorption should have celiac panels, vitamin levels, mineral levels run (and probably mannitol tests for malabsorption as well). Trying to get doctors to connect the dots, though, is difficult.