Vitamin B12 Blood Test
Feb 4, 2026
Vitamin B12 Blood Test
A vitamin B12 blood test measures your cobalamin levels—essential for energy production, brain function, nerve health, and DNA synthesis, with deficiency affecting up to 6% of adults under 60 and around 20% of those over 60. B12 deficiency can cause fatigue, memory problems, and nerve damage that may become permanent if untreated.
At Blood Test London, B12 testing costs from £245 with results in 2-3 days.
What Does a B12 Test Measure?
Test | What It Shows | Normal Range |
|---|---|---|
Serum B12 | B12 in blood | 200-900 pg/mL (150-675 pmol/L) |
Active B12 | Usable B12 (more accurate) | 37-188 pmol/L |
MMA (methylmalonic acid) | Metabolic marker of deficiency | Below 280 nmol/L |
The "Normal" Problem
Standard B12 ranges are controversial. Many experts consider levels below 400-500 pg/mL as potentially suboptimal, even though labs may report them as "normal."
Why B12 Is Crucial
Energy Production
B12 helps convert food into cellular energy. Deficiency causes profound fatigue.
Brain and Nerve Function
Essential for myelin (nerve coating) and neurotransmitter production. Deficiency can cause:
Memory problems
Confusion
Depression
Peripheral neuropathy (tingling, numbness)
Blood Cell Formation
Works with folate to produce red blood cells. Deficiency causes megaloblastic anaemia.
DNA Synthesis
Required for cell division throughout the body.
Signs of B12 Deficiency
Physical Symptoms
Extreme fatigue
Weakness
Pale or yellowish skin
Shortness of breath
Heart palpitations
Mouth ulcers
Glossitis (sore, red tongue)
Neurological Symptoms
Tingling or numbness in hands/feet
Difficulty walking or balance problems
Memory problems
Confusion
Depression
Vision changes
Who's Most at Risk?
Vegans and vegetarians (B12 is mainly in animal products)
Over 60s (reduced absorption)
Those on metformin (diabetes medication)
Those on PPIs (acid reflux medication)
People with digestive conditions (Crohn's, coeliac, gastric surgery)
Pernicious anaemia (autoimmune condition affecting absorption)
How to Optimise B12
Food Sources
Food | B12 per serving |
|---|---|
Liver (75g) | 52 μg |
Clams (85g) | 84 μg |
Beef (85g) | 2.1 μg |
Salmon (85g) | 4.8 μg |
Eggs (2 large) | 1.1 μg |
Milk (250ml) | 1.1 μg |
Fortified nutritional yeast | Varies |
Daily requirement: 2.4 μg (but absorption limits mean more is needed from food)
Supplementation
Level | Action |
|---|---|
Above 500 pg/mL | Maintain with diet |
300-500 pg/mL | Consider supplementation |
Below 300 pg/mL | Supplement recommended |
Below 200 pg/mL | Higher doses or injections may be needed |
Forms: Methylcobalamin or adenosylcobalamin preferred over cyanocobalamin.
For Vegans
Supplementation is essential—there are no reliable plant sources of B12.
Book Your B12 Test
Price: From £245
Results: 2-3 days
Doctor review: A doctor always reviews every result
Fasting required: No
FAQ
What B12 level is optimal?
Many experts recommend above 500 pg/mL, though labs may show lower as "normal."
Can B12 deficiency be dangerous?
Yes—neurological damage can become permanent if severe deficiency is untreated.
How long to correct deficiency?
Symptoms often improve within weeks of supplementation, though nerve damage may take longer.
Can I take too much B12?
B12 is water-soluble and excess is excreted. Toxicity is extremely rare.
Blood Test London by The Wellness. B12 testing from £245, results in 2-3 days.