Vitamin B12, or cobalamin, is essential for red blood cell formation, DNA synthesis, and neurological function. In the UK, B12 levels are measured in picomoles per litre (pmol/L), with normal values typically above 258 pmol/L. Understanding what constitutes a normal B12 number is crucial, as deficiency can lead to serious complications including irreversible neurological damage. Reference ranges vary between laboratories, so results must be interpreted alongside clinical symptoms and other blood parameters. This article explains UK reference ranges, how to interpret your results, and when to seek medical advice.

What Is a Normal B12 Number in the UK?

In the United Kingdom, vitamin B12 levels are typically reported in picomoles per litre (pmol/L), though some laboratories may use nanograms per litre (ng/L). The reference ranges can vary between laboratories due to differences in testing methodology and equipment calibration.

According to NICE Clinical Knowledge Summaries (CKS), the following thresholds are used for interpreting vitamin B12 results:

• Deficiency: less than 148 pmol/L (approximately 200 ng/L)

• Borderline: between 148-258 pmol/L (approximately 200-350 ng/L)

• Normal: above 258 pmol/L, with upper limits varying by laboratory

It is important to check the specific reference range provided on your blood test report, as this may differ slightly between NHS trusts and private laboratories.

Symptoms of B12 deficiency can sometimes occur even when levels fall within the borderline range, which is why clinical correlation with symptoms is essential. Your doctor will interpret your result in the context of your symptoms and other blood test findings.

Vitamin B12, also known as cobalamin, is a water-soluble vitamin essential for red blood cell formation, DNA synthesis, and neurological function. Measuring serum B12 is the first-line investigation when deficiency is suspected, though additional tests such as methylmalonic acid (MMA) or homocysteine levels may be requested if results are borderline or symptoms persist despite apparently normal levels.

Understanding Your B12 Blood Test Results

When you receive your B12 blood test results, the report will typically show your measured level alongside the laboratory's reference range. Understanding what these numbers mean requires consideration of both the numerical value and your clinical presentation.

Serum B12 measurement is performed using a venous blood sample, usually taken in the morning, though fasting is not typically required. The test measures the total amount of vitamin B12 circulating in your bloodstream, bound to transport proteins called transcobalamins. However, this total measurement does not always reflect the amount of B12 actually available for cellular use, which is why some patients experience deficiency symptoms despite borderline-normal results.

Your GP will interpret your B12 level in conjunction with other blood parameters, particularly:

• Full blood count (FBC) – looking for macrocytic anaemia (enlarged red blood cells) or reduced haemoglobin

• Mean corpuscular volume (MCV) – elevated MCV suggests B12 or folate deficiency

• Blood film – may show hypersegmented neutrophils characteristic of B12 deficiency

• Folate levels – often checked simultaneously as deficiencies can coexist

If your B12 level is borderline (148-258 pmol/L) and you have suggestive symptoms, your doctor may request additional tests. Methylmalonic acid (MMA) and homocysteine are metabolites that accumulate when B12 is functionally deficient at the cellular level. Elevated levels of these markers can help confirm true B12 deficiency even when serum B12 appears adequate. However, it's important to note that these markers can also be elevated in other conditions, particularly renal impairment, so results must be interpreted carefully.

If pernicious anaemia is suspected, your doctor may test for anti-intrinsic factor antibodies after a low B12 result is confirmed. Some specialists also measure holotranscobalamin (active B12), which represents the biologically available fraction, though this test is not yet routinely available across all NHS services.

What Do Low or High B12 Levels Mean?

Low B12 levels (below 148 pmol/L or approximately 200 ng/L) indicate vitamin B12 deficiency, which can result from several underlying causes. Common causes in the UK include pernicious anaemia (an autoimmune condition where the stomach fails to produce intrinsic factor, a protein essential for B12 absorption) and dietary deficiency, particularly in those following strict vegan diets (as B12 is found primarily in animal products).

Other causes include malabsorption disorders such as Crohn's disease or coeliac disease, gastric surgery, and certain medications. The MHRA has highlighted that long-term metformin use can reduce B12 absorption, and prolonged use of proton pump inhibitors (PPIs) may also affect B12 status.

Symptoms of B12 deficiency develop gradually and may include:

• Persistent fatigue and weakness

• Neurological symptoms such as paraesthesia (pins and needles), particularly in hands and feet

• Cognitive changes including memory problems or confusion

• Glossitis (sore, red tongue) and mouth ulcers

• Mood disturbances including depression or irritability

• Balance problems and difficulty walking

Neurological complications can become irreversible if left untreated, making prompt diagnosis and treatment essential.

Elevated B12 levels (above the laboratory's upper reference limit) warrant clinical consideration. While excess water-soluble B12 from supplements is generally excreted, unexpectedly high B12 levels without supplementation may indicate underlying conditions requiring investigation. These include liver disease (where B12 is released from damaged liver cells), renal impairment, myeloproliferative disorders, certain malignancies, or excess binding proteins. If your B12 is significantly elevated without supplementation, your GP may investigate further to identify potential underlying causes.

If you experience side effects that you suspect may be related to a medicine (such as metformin-associated B12 deficiency), you can report this through the MHRA Yellow Card Scheme.

When to Get Your B12 Levels Checked

You should consider requesting a B12 blood test if you experience symptoms suggestive of deficiency, particularly if you have risk factors. NICE guidance recommends testing for B12 deficiency in patients presenting with unexplained anaemia, neurological symptoms, or macrocytosis on routine blood tests.

Specific situations warranting B12 testing include:

• Persistent unexplained fatigue or weakness

• Neurological symptoms such as numbness, tingling, or balance problems

• Cognitive changes or memory difficulties

• Following a diagnosis of autoimmune conditions (pernicious anaemia often coexists with other autoimmune diseases)

• Adherence to a strict vegan or vegetarian diet without supplementation

• Gastrointestinal disorders affecting absorption (Crohn's disease, coeliac disease, previous gastric surgery)

• Long-term use of medications that interfere with B12 absorption, including metformin (commonly prescribed for type 2 diabetes) and PPIs

• Unexplained macrocytic anaemia detected on routine blood tests

• Pregnancy, where prompt treatment of deficiency is important

Urgent action is needed if you develop neurological symptoms such as significant paraesthesia, ataxia (poor coordination), gait disturbance, or cognitive changes. These may indicate subacute combined degeneration of the spinal cord, which requires immediate treatment with intramuscular hydroxocobalamin injections and possibly specialist referral.

Routine screening is not recommended for the general population. However, individuals following plant-based diets should be particularly vigilant, as dietary B12 is predominantly found in meat, fish, dairy, and eggs. Vegans should either consume fortified foods regularly or take B12 supplements, with periodic monitoring if symptoms develop.

If deficiency is confirmed, treatment in the UK typically involves intramuscular hydroxocobalamin injections. The standard regimen is 1 mg three times weekly for two weeks for those without neurological involvement. For those with neurological symptoms, more intensive treatment (alternate days until improvement) may be required. For pernicious anaemia, lifelong maintenance injections (typically every 2-3 months) are needed. Oral supplementation may be appropriate for dietary deficiency in the absence of malabsorption. Your GP will discuss the most suitable treatment regimen based on the underlying cause and severity of your deficiency.

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