Vitamin B12
Serum Vitamin B12
What this test measures
Vitamin B12 helps your body make red blood cells and keeps your nerves healthy. You get it from foods like meat, fish, eggs, and dairy. This test checks if you have enough B12 in your blood.
Understanding the numbers
Typical range: 148-600 pmol/L
Below 148 pmol/L may indicate deficiency. Symptoms can occur even with levels in the 'normal' range. Ranges may vary slightly between laboratories.
- Deficient: 50–148 pmol/L
- Grey zone: 148–250 pmol/L
- Normal: 250–600 pmol/L
- Raised: 600–1500 pmol/L
Key numbers
- 148 pmol/L — Below = deficient, treat
- 250 pmol/L — Symptomatic patients often benefit from treatment below 250
British Society for Haematology (2014) recommends treating B12 below 148 pmol/L; values 148-250 with clinical or haematological features of deficiency often justify a trial of treatment. A normal or high serum B12 does not exclude deficiency — consider MMA (methylmalonic acid) if clinical suspicion is high.
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean
- High B12 is usually not harmful
- Can happen if you take B12 supplements or have B12 injections
- Very high levels without supplements may need investigation
- Your GP will advise if any action is needed
Questions you could ask your GP
- Do I need B12 tablets or injections, and for how long?
- What is causing my B12 deficiency - could it be my diet or a gut problem?
- Should I be tested for pernicious anaemia?
- Will I need B12 treatment for life?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
Vegan with pins and needles
Nabila had been vegan for eight years. She had tingling in her feet and brain fog. B12 105 pmol/L.
What the GP said: “This is dietary B12 deficiency — strict vegan diets can't provide enough without supplementation. Because you have neurological symptoms, we'll start with injections to load, then you can stay on high-dose oral B12 long-term or continue injections.”
What happened next: After loading injections the tingling settled. Nabila continued with 1000 µg oral B12 daily.
Dietary B12 deficiency responds to replacement (injections or high-dose oral). Neurological symptoms need loading injections to protect the nerves — oral alone isn't adequate for early treatment.
Pernicious anaemia
Pam was tired and had a sore tongue. B12 84, haemoglobin 98 with large cells (MCV 112), intrinsic factor antibodies positive.
What the GP said: “This is pernicious anaemia — an autoimmune condition where the stomach can't absorb B12. You need B12 injections for life, usually every three months after a loading course.”
What happened next: After loading, her haemoglobin rose to 132 and she felt much better.
Pernicious anaemia is lifelong. Oral tablets don't work because the absorption pathway is damaged — injections are needed every three months. Intrinsic factor antibodies help confirm it.
Metformin — mild B12 deficiency
Marlon has Type 2 diabetes on metformin for 8 years. B12 140. He had mild tingling.
What the GP said: “Long-term metformin lowers B12 absorption. A daily oral B12 supplement 1000 µg should bring it back up. We can continue metformin.”
What happened next: After three months on oral B12, his level was 320 and tingling settled.
Long-term metformin (and PPIs) reduce B12 absorption. Screening B12 on metformin, especially with any neurological symptoms, is worthwhile.
High B12 — investigating raised level
Harminder's B12 was 1250 on a routine check. He was not on supplements. He had no symptoms.
What the GP said: “High B12 without supplements deserves a look. Liver disease, some blood disorders and rarely certain cancers can cause it. Let's check your liver tests, kidney function and blood count.”
What happened next: Further tests revealed fatty liver; once treated with lifestyle changes, B12 came down.
Raised B12 without supplementation has been linked in some studies to liver disease, blood conditions and (rarely) malignancy. It deserves a sensible workup rather than being ignored.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.