Vitamin B12 Deficiency Interpreter: Low, Borderline or Normal B12
Vitamin B12 Deficiency Interpreter: Low, Borderline or Normal B12
Decide whether a serum B12 result means deficiency. Enter the B12 and your laboratory’s lower limit, the active B12 (holotranscobalamin) and methylmalonic acid if measured, and the context — neurological symptoms, pregnancy, the oral contraceptive, nitrous oxide, metformin, acid suppression, a vegan diet — and the page says whether deficiency is likely, indeterminate or unlikely on the BSH 2014 and NICE 2024 guidelines, and where a normal result must not stop treatment.
Is this B12 deficiency?
Serum B12 vs lab limit + MMA, active B12, symptoms, context → likely, indeterminate, unlikelyA 64-year-old on metformin for 8 years is tired. Serum B12 190 pmol/L, laboratory lower limit 148 pmol/L; no MMA or active B12; no neurological or blood film abnormality.
How the two guidelines read a B12
NICE NG239: total B12 < 133 pmol/L (180 ng/L) or active B12 < 25 pmol/L = confirmed; 133–258 pmol/L (180–350 ng/L) or 25–70 = indeterminate; above those = unlikely
Either guideline: strong clinical features, especially neurological, are not overruled by a normal result
- Active B12
- holotranscobalamin, the fraction delivered to cells; NICE’s first-line test in pregnancy
- MMA
- methylmalonic acid; rises when B12 cannot act; also raised by kidney disease
- pmol/L ↔ ng/L
- ng/L = pmol/L × 1.355 (cobalamin, 1355 g/mol)
Worked example
A 64-year-old on metformin for 8 years is tired. Serum B12 190 pmol/L, laboratory lower limit 148 pmol/L; no MMA or active B12; no neurological or blood film abnormality.
190 pmol/L = 258 ng/L; 190 ÷ 148 = 1.28 × the laboratory's lower limit → normal on the BSH approach
NICE NG239: 133–258 pmol/L is indeterminate → possible deficiency
The two guidelines disagree here; with only non-specific symptoms, NICE advises returning if symptoms develop, and MMA if they do
Where the BSH and NICE differ
| Question | BSH 2014 | NICE NG239 (2024) |
|---|---|---|
| Deficiency threshold | 148 pmol/L (200 ng/L) or local limit | Below 133 pmol/L (180 ng/L) = confirmed |
| Indeterminate | Within 25% below the lower limit (about 110–148 pmol/L) | 133–258 pmol/L (180–350 ng/L); active B12 25–70 pmol/L |
| First-line test | Serum B12; holotranscobalamin suggested for the future | Total or active B12; active B12 in pregnancy; homocysteine or MMA after nitrous oxide |
| Second-line test | MMA or homocysteine; MMA more specific | MMA, using the laboratory’s reference range |
| Normal result, strong features | Do not delay treatment | Do not delay in suspected megaloblastic anaemia with neurological symptoms |
Why a normal B12 does not exclude deficiency
Serum B12 is one of the most requested tests in medicine and one of the least decisive. It measures total cobalamin, most of which is bound to haptocorrin and unavailable to cells, so it can be normal in real deficiency and low in people who are not deficient — in pregnancy, on the oral contraceptive, and in many healthy people with low haptocorrin. The British Society for Haematology’s 2014 guideline (Devalia et al.) puts the clinical picture first: when strong clinical features and the test disagree, treatment should not be delayed, to avoid neurological impairment. A quarter of patients with B12 neuropathy have a normal MCV, so a normal blood count does not exclude it either.
NICE’s 2024 guideline (NG239) chose deliberately high cut-offs because people are usually tested for symptoms and treatment is cheap and safe. It calls a total B12 below 180 ng/L (133 pmol/L) confirmed deficiency, 180 to 350 ng/L (133 to 258 pmol/L) indeterminate, and above that unlikely — so a result of 200 pmol/L is normal on most laboratory reports and indeterminate on NICE. NICE also makes active B12 the first test in pregnancy, homocysteine or MMA the first test after nitrous oxide, and warns that the pill lowers total B12 without deficiency.
Methylmalonic acid shows whether B12 is working in the tissues, but reduced kidney function raises it. Nitrous oxide inactivates B12 without lowering the serum level. High-titre intrinsic factor antibodies can make some assays report a falsely normal B12. Convert units with the vitamin B12 unit converter, the methylmalonic acid unit converter and the homocysteine unit converter. This interpreter supports, and does not replace, clinical judgement.
Frequently asked questions
Can you have B12 deficiency with a normal B12 level?
Yes. Serum B12 measures total cobalamin, much of it unavailable to cells, and intrinsic factor antibodies can make some assays read falsely normal. The BSH 2014 guideline says treatment should not be delayed when strong clinical features disagree with the result.
What B12 level is borderline?
On NICE NG239, 180 to 350 ng/L (133 to 258 pmol/L), or an active B12 of 25 to 70 pmol/L. On the BSH approach, within about 25% below the laboratory’s lower limit — roughly 110 to 148 pmol/L.
When is methylmalonic acid useful?
When the B12 is indeterminate and deficiency is suspected. A raised MMA shows B12 is not acting in the tissues; the BSH calls it more specific than homocysteine, but kidney disease also raises it.
Should neurological symptoms be treated before the results come back?
Yes. NICE NG239 says not to delay replacement while waiting for results in suspected megaloblastic anaemia with neurological symptoms, and the BSH says treatment should not be delayed when strong clinical features and the test disagree. Nerve damage treated late may not recover.
Related calculators
References
- Devalia V, Hamilton MS, Molloy AM; British Committee for Standards in Haematology. Guidelines for the diagnosis and treatment of cobalamin and folate disorders. Br J Haematol. 2014;166(4):496–513.
- National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management. NICE guideline NG239. March 2024. Table 1 and recommendations 1.2–1.4.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/