The National Library of Medicine lists the normal B-12 level for a healthy adult as anywhere from 200 to 900 picograms per milliliter. Levels below 200 picograms per milliliter generally indicate a deficiency. Levels above 900 picograms per milliliter are uncommon as excess B-12 is released during urination.
Normal B-12 levels vary based on the lab's specific testing procedures and B-12 tests are mainly used in conjunction with other blood tests to diagnose deficiencies, according to the National Library of Medicine. The specific cause of a B-12 deficiency can be determined using a Schilling test. In most cases, it's due to malabsorption or dietary issues. Older adults may suffer from a B12 deficiency even with levels as high as 500 picograms per milliliter.