Normal Vitamin B12 Levels
Normal vitamin B12 levels range from 180-350 ng/L (indeterminate range) to above 350 ng/L, with levels below 180 ng/L indicating confirmed vitamin B12 deficiency according to the most recent guidelines. 1
Interpretation of Vitamin B12 Levels
Serum B12 Interpretation
- Less than 180 ng/L: Confirmed vitamin B12 deficiency 1
- 180-350 ng/L: Indeterminate result requiring further testing 1
- More than 350 ng/L: Vitamin B12 deficiency unlikely 1
Holotranscobalamin (Active B12) Interpretation
- Less than 25 pmol/L: Confirmed vitamin B12 deficiency 1
- 25-70 pmol/L: Possible vitamin B12 deficiency 1
- More than 70 pmol/L: Vitamin B12 deficiency unlikely 1
Confirmatory Testing
When serum B12 levels fall in the indeterminate range (180-350 ng/L), additional testing is recommended:
- Methylmalonic acid (MMA): Levels >271 nmol/L strongly suggest functional vitamin B12 deficiency, even when serum B12 appears normal 1
- Holotranscobalamin (Active B12): More sensitive marker of B12 status 2
- Homocysteine: Elevated in B12 deficiency 1
Important Considerations
Limitations of Serum B12 Testing
- Studies show that 15-50% of patients with normal serum B12 may have metabolic deficiency 1
- Relying solely on serum B12 without confirmatory testing can lead to missed diagnoses 1
Optimal Range for Function
- Research suggests that vitamin B12 levels between 400-700 pg/mL may be optimal for hemoglobin synthesis in athletes 3
- Significant increases in hemoglobin appear up to approximately 400 pg/mL, with no significant changes above 700 pg/mL 3
Dietary Recommendations
- UK Reference Nutrient Intake (RNI): 1.5 μg/day for adults 2
- European Food Safety Authority (EFSA): 4 μg/day 2
- German Nutrition Society: 3 μg/day 2
- Recent comprehensive analyses suggest 4-7 μg/day may be required for normal functional markers 2
Special Populations
Vegetarians and Vegans
- Higher risk of B12 deficiency: 92% of vegans, 64% of lactovegetarians, and 47% of lacto-ovovegetarians without supplementation may have serum B12 levels below 200 pg/mL 4
- Supplementation of 50 μg/day (350 μg/week) of sublingual cobalamin can effectively restore adequate B12 levels in those with marginal deficiency 5
Elderly Patients
- More susceptible to B12 deficiency due to decreased absorption
- Helicobacter pylori infection is significantly more common in elderly patients with low B12 levels (≤200 pg/mL) 6
Treatment Thresholds
When B12 deficiency is confirmed:
- For patients with neurological involvement: Hydroxocobalamin 1 mg intramuscularly on alternate days until no further improvement, then 1 mg every 2 months 2
- For patients without neurological involvement: Hydroxocobalamin 1 mg intramuscularly three times a week for 2 weeks, followed by 1 mg every 2-3 months for life 2
Remember that folic acid supplementation can mask B12 deficiency hematologically but not prevent neurological damage, so B12 status should always be confirmed before initiating folate treatment 2, 1.