Vitamin B12 Blood Test: Deficiency, MMA, and When "Normal" Is Not Enough
Serum B12 misses up to 25% of true deficiency. Learn when to add methylmalonic acid, holotranscobalamin, and homocysteine to confirm.
B12 deficiency causes megaloblastic anaemia AND neurological damage that can precede anaemia. Serum B12 < 148 pmol/L (< 200 pg/mL) is deficient. Borderline 148–221 pmol/L is common and unreliable — confirm with methylmalonic acid (MMA) or holotranscobalamin (active B12). Treat all symptomatic patients even with borderline levels — neurological damage may be irreversible if delayed.
Vitamin B12 (cobalamin) deficiency affects up to 20% of older adults but is under-diagnosed because serum total B12 is an imperfect marker. Symptoms include fatigue, pallor, glossitis, paraesthesias, unsteady gait, memory impairment, and (rarely) subacute combined degeneration of the cord. Common causes: pernicious anaemia (autoimmune, anti-intrinsic-factor and anti-parietal-cell antibodies), veganism without supplementation, atrophic gastritis (older adults), long-term PPI or metformin use, ileal disease or resection. NICE 2024 guideline emphasises treating symptomatic patients even at borderline B12 (148–221 pmol/L) rather than delaying. Confirmatory MMA or active B12 (holotranscobalamin) resolves 80% of borderline cases. Treatment: parenteral hydroxocobalamin for pernicious anaemia or malabsorption; oral 1000 mcg/day is adequate for dietary deficiency.
- Deficient
- < 148 pmol/L (< 200 pg/mL)
- Borderline
- 148 – 221 pmol/L
- Confirmatory
- MMA or holotranscobalamin
- Parenteral dose
- Hydroxocobalamin 1 mg IM alternate days × 6
- Oral dose
- 1000 mcg/day if dietary
B12 result interpretation
| B12 (pmol/L) | B12 (pg/mL) | Interpretation | Next step |
|---|---|---|---|
| < 148 | < 200 | Deficient | Treat + investigate cause |
| 148 – 221 | 200 – 300 | Borderline | MMA or active B12 confirmation |
| 221 – 665 | 300 – 900 | Normal | No action |
| > 665 | > 900 | High | Usually benign; occasionally liver/haem malignancy |
Confirmatory testing when borderline
Methylmalonic acid (MMA): rises when B12 is functionally low. MMA > 0.37 µmol/L strongly supports deficiency.
Holotranscobalamin (active B12): the biologically active fraction — more sensitive than total B12. < 25 pmol/L = deficient.
Homocysteine: rises in B12 and folate deficiency; useful but less specific.
- B12 low + neurological symptoms (paraesthesias, unsteady gait, memory loss) — do not wait for confirmatory tests.
- Macrocytic anaemia (MCV > 100 fL) + low B12 — treat immediately.
- B12 normal but neurological signs persist — check MMA/active B12.
B12 workup
- 1B12 < 148 pmol/L?Deficient. Treat. Investigate cause: anti-intrinsic-factor antibody, dietary review, PPI/metformin history.
- 2B12 148–221 + symptoms?Check MMA or active B12. Consider empirical treatment if strongly symptomatic.
- 3Positive anti-intrinsic-factor antibody?Pernicious anaemia — lifelong parenteral B12.
- 4Vegan/vegetarian?Oral 1000 mcg/day cyanocobalamin or fortified foods.
Related questions people ask
- What is MMA and when to order it?
- How does homocysteine relate to B12?
- Does CBC help diagnose B12 deficiency?
- Iron deficiency co-existing?
Frequently asked questions
- Treat symptomatic B12 deficiency even at borderline levels.
- MMA or active B12 confirms borderline results.
- Metformin, PPIs, veganism, and pernicious anaemia are the top causes.
- Neurological symptoms may be irreversible — do not delay.
- Parenteral for pernicious anaemia; oral for dietary.
References
2 sources- NICE NG239B12 Deficiency in Adults (2024)
UK national guideline.
nice.org.uk
- BSHDiagnosis and Treatment of Cobalamin Deficiency
British Society for Haematology.
onlinelibrary.wiley.com
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