Vitamin B12
Also known as: Cobalamin, Cyanocobalamin
Vitamin B12 is essential for nerve function, DNA synthesis, and red blood cell production. Deficiency develops slowly — often over years — and can cause irreversible neurological damage before blood cell changes appear.
Venous blood (serum separator tube)
24–48 hours
Not required
Vitamins & Minerals
Reference Ranges
| Measurement | Reference Range | Unit |
|---|---|---|
| Deficiency | ≤ 200 | pg/mL |
| Grey zone | 200 – 300 | pg/mL |
| Sufficient | ≥ 300 | pg/mL |
Reference ranges are typical adult values. Your laboratory may use slightly different intervals — always use your lab's reported reference.
What This Test Measures
The serum B12 test measures total cobalamin circulating in the blood, including inactive forms. Because roughly 20–30% of serum B12 is inactive, some people with 'normal' B12 levels still have functional deficiency — where active B12 is insufficient for metabolic needs. Methylmalonic acid (MMA) and homocysteine are more sensitive functional markers.
When Is This Test Ordered?
- Investigating macrocytic anaemia (large red blood cells)
- Peripheral neuropathy or unexplained neurological symptoms
- Cognitive decline or memory problems
- Monitoring vegans, vegetarians, and breastfeeding mothers
- Patients on long-term metformin or proton pump inhibitors
- After gastric surgery or with atrophic gastritis
When Results Are High
Very high B12 (above 1,500 pg/mL) in someone not supplementing may indicate liver disease, myeloproliferative disorders, or solid tumours. It is not a sign of toxicity from diet.
When Results Are Low
Below 200 pg/mL indicates deficiency. Early symptoms: fatigue, glossitis (sore tongue), subtle memory changes. Advanced deficiency causes subacute combined degeneration of the spinal cord — irreversible numbness, balance problems, and dementia — making early detection critical.
Who is at risk of B12 deficiency?
- Vegans and strict vegetarians — B12 exists only in animal-derived foods
- Adults over 60 — gastric acid and intrinsic factor secretion decline with age
- Metformin users — drug reduces intestinal B12 absorption
- Long-term PPI users — gastric acid needed to release food-bound B12
- Pernicious anaemia — autoimmune destruction of parietal cells that produce intrinsic factor
- Post-gastric bypass or gastrectomy patients
Factors That Can Affect Your Result
- B12 supplements or injections within the past week falsely raise the result
- Metformin reduces B12 absorption by 20–30% over years of use — screen diabetics annually
- Proton pump inhibitors impair the acid needed to release B12 from food
- Intrinsic factor deficiency (pernicious anaemia) blocks intestinal absorption regardless of dietary intake
- Folate deficiency and B12 deficiency both cause macrocytosis — check both together
Tracking This Test Over Time
If your B12 is in the grey zone (200–300 pg/mL) and you have symptoms, ask for methylmalonic acid testing — it is a more sensitive marker of functional deficiency. A raised MMA confirms B12 is functionally insufficient even when the serum level looks normal.
Track your B12 in LoomsRelated Tests
Folate
Folate is essential for DNA synthesis and cell division — it is critically important in early pregnancy for preventing neural tube defects, and together with B12 it prevents megaloblastic anaemia.
CBC
A CBC measures all three types of blood cells — red cells, white cells, and platelets — along with haemoglobin and haematocrit. It is one of the most ordered tests in medicine because it screens for dozens of conditions in a single draw.
Medical disclaimer
This page is for educational purposes only and does not constitute medical advice. Reference ranges vary between laboratories, and results must be interpreted in the context of your symptoms, history, and other tests. Always discuss your results with your healthcare provider. Content reviewed by the Looms medical team, 2026-06-01.