Vitamin B12

Also written Cobalamin

NutrientsMeasured in pg/mL

What it measures

Total serum B12, most of which is bound to a protein that does not deliver it to tissues.

Why it matters

Deficiency causes anemia and potentially irreversible neurological injury, and it is common in older adults, vegans, and long-term metformin or acid-suppression users.

Where the claimed ranges actually sit

Measured in pg/mL. Each row is one claim about where your result should fall.

What a laboratory reportsWhat a seller markets as “optimal”

Laboratory reference range200–900 pg/mL

Laboratory reference intervals

Above 500 pg/mLC> 500 pg/mL

B-vitamin supplement marketing and functional medicine

›View as table

Claim	Range	Source
Laboratory reference range	200–900 pg/mL	Laboratory reference intervals
Above 500 pg/mL	> 500 pg/mL	B-vitamin supplement marketing and functional medicine

Where the sources disagree

Both sides have a point, and the resolution is a different test rather than a different threshold. The conventional lower bound genuinely misses tissue deficiency in some people; the commercial answer of supplementing everyone under 500 is cheap and mostly harmless but rarely necessary. Methylmalonic acid settles the question directly, and it is inexpensive.

The marketed ranges, examined

Each claim gets its strongest possible case before it gets its criticism. Several of these are substantially correct.

Above 500 pg/mL

Promoted by b-vitamin supplement marketing and functional medicine

CConfounded

The strongest case for it

Serum B12 is an insensitive test, and people with values in the 200–400 range can have genuine tissue deficiency detectable via methylmalonic acid. Neurological damage from B12 deficiency can be irreversible, which argues for caution.

What it leaves out

Rather than raising the cutoff, the correct response to a borderline B12 is to measure methylmalonic acid, which directly reflects tissue adequacy. Supplementing everyone below 500 treats the insensitivity of the assay with a product instead of a better test.

Before you act on your result

Serum B12 is insensitive; confirm borderline results with methylmalonic acid.
Metformin and long-term acid suppression reduce absorption.
High-dose biotin interferes with many B12 immunoassays.

Last reviewed July 28, 2026How we grade evidence and attribute claims →

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