B12 builds red blood cells and insulates nerves. Its deficiency creeps in over years, and the neurological damage can become permanent while the blood count still looks fixable.
Standard range versus optimal range
| Framework | Vitamin B12 value | Source |
|---|---|---|
| Deficiency likely | below 200 pg/mL | Stabler, NEJM 2013 |
| Borderline, confirm with MMA | 200-300 pg/mL | Stabler, NEJM 2013 |
| Sufficient | above 300-400 pg/mL | Stabler, NEJM 2013 |
When it runs high
High B12 is usually just supplementation, since excess is excreted. Persistently high values without supplements occasionally signal liver disease or blood disorders and deserve a look.
- Supplements and injections, the usual cause
- Liver disease releasing stored B12
- Myeloproliferative blood disorders, rare
When it runs low
Low B12 produces large-cell anemia and a characteristic nerve injury, tingling feet, poor balance and memory slips. The nervous system damage can outrun the blood changes, so symptoms matter more than the number.
- Pernicious anemia, autoimmune stomach attack
- Long-term metformin or acid-blocking drugs
- Strict vegan diet without supplementation
- Stomach or bowel surgery
- Age-related absorption decline
- Celiac or Crohn disease
The slow leak vitamin
The liver stores several years of B12, which makes deficiency a story of slow motion. Absorption fails first, through autoimmune stomach damage, metformin, acid blockers or age, and the tank drains silently for years before anemia appears. Nerves often complain earliest, tingling that starts in the toes and creeps upward, because the myelin insulation around nerve fibers depends on B12. Treat late and that damage stays, treat early and it reverses, which is why borderline results in symptomatic people deserve the confirmatory tests.
Beyond the basic number
Serum B12 misses cases because much of what it measures rides on an inactive carrier. Two better witnesses exist, methylmalonic acid, which rises when cells run out of B12, and holotranscobalamin, which measures only the active fraction. A normal B12 with high MMA is a deficiency the basic test missed, a pattern especially common in older adults and metformin users.
Treatment is cheap and forgiving
High-dose oral B12 works surprisingly well even with broken absorption, because about one percent diffuses passively. Injections remain the classic route for pernicious anemia and neurological symptoms. Either way, retest in two to three months, expect the blood picture to normalize within weeks and nerve symptoms to improve over months.
Related calculators
Related biomarkers Folate · MMA · HoloTC · Homocysteine · MCV