Vitamin B12 blood test: normal levels and deficiency

Vitamin B12 levels: why a normal result may not rule out deficiency, the gray zone from 180 to 350 pg/mL and the tests that can settle it.

6 min - BioSense Team - 2026-10-16

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A vitamin B12 level is measured with a simple blood test, but a result "within range" does not always rule out a deficiency. In the US, a 2025 American Family Physician review treats a level below 180 pg/mL (133 pmol/L) as diagnostic and calls for further testing between 180 and 350 pg/mL (133 and 258 pmol/L). Here is what that test measures, why one band of results stays ambiguous, and which signs come before anemia.

You have been tired for months. You have tingling in your hands or feet. Your doctor ordered a vitamin B12 test. The result came back at 280 pg/mL (207 pmol/L), inside the range shown on your report.

The matter is settled, apparently. In reality, that figure sits in a zone that specialists themselves describe as uncertain.

What does vitamin B12 do?

Vitamin B12, also called cobalamin, is involved in two vital functions.

It takes part in making DNA, and therefore in cell multiplication. The cells that renew themselves fastest depend on it first, in particular those in the bone marrow that produce red blood cells.

It also maintains the sheath that protects the nerves. When it runs short, that sheath degrades progressively, in the brain and the spinal cord. This shows up as disturbances of sensation and of movement.

Our body cannot make it. It is produced by bacteria and reaches us naturally through animal products: meat, organ meats, fish, eggs, dairy. Fortified foods and supplements also provide it. In the US, the Recommended Dietary Allowance is 2.4 mcg per day for adults, 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding.

A normal B12 level does not rule out a deficiency

The British Society for Haematology guideline (2014) states it bluntly: there is no "gold standard" test that defines B12 deficiency. The blood test remains the first-line test, but its result cannot be read on its own.

B12 below 180 to 200 pg/mL: the deficiency range

Two cutoffs are in circulation, and they are close without being identical:

In the US, the 2025 review in American Family Physician treats a total B12 below 180 pg/mL (133 pmol/L) as diagnostic. The NIH Office of Dietary Supplements notes that most laboratories define low values as below 200 or 250 pg/mL (148 or 185 pmol/L).
In the UK, the NICE guideline (2024) considers deficiency confirmed below 180 ng/L (133 pmol/L). The older British Society for Haematology guideline (2014) used 200 ng/L (148 pmol/L). One ng/L is the same as one pg/mL.

B12 between 180 and 350 pg/mL: the gray zone

Above that, things get more complicated. American Family Physician and NICE both treat 180 to 350 pg/mL (133 to 258 pmol/L) as borderline or indeterminate. NICE adds that deficiency is unlikely above 350 pg/mL (258 pmol/L). MedlinePlus goes further and describes a value below 299 pg/mL (219 pmol/L) as a possible sign of deficiency, to be confirmed with methylmalonic acid.

In other words, a result at 280 pg/mL (207 pmol/L) rules nothing out. Nor does it confirm anything. It needs to be read in context: your symptoms, your age, your treatments, your diet.

B12 testing: every lab has its own reference range

Assay techniques vary from one analyzer to another, and the British guideline specifies that universal cutoffs are not possible. Each laboratory establishes its own reference range. MedlinePlus lists 299 to 1,054 pg/mL (219 to 773 pmol/L), whereas the January 2026 ABIM reference ranges list 200 to 800 pg/mL (148 to 590 pmol/L). Comparing a 2024 result from one laboratory with a 2026 result from another therefore calls for caution.

Key point - There is no reference test for B12 deficiency. Below 180 pg/mL (133 pmol/L), deficiency is likely. Between 180 and 350 pg/mL (133 and 258 pmol/L), the result does not settle the question and must be read alongside the symptoms and the context.

This is precisely what BioSense does. Import your blood test, answer a few questions about your situation (your symptoms, your treatments, your diet, the reason for the test), and receive a reading of each biomarker that takes that context into account. Your data is hosted on servers located in France.

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B12 deficiency: why a normal MCV is not enough

One idea circulates widely: that a B12 deficiency would show up on the complete blood count (CBC), because it makes red blood cells larger. That volume is called the MCV, for mean corpuscular volume.

That is true at an advanced stage. It is not true at the start.

The British guideline is explicit on this point. The abnormalities visible on the blood count, large red blood cells and so-called hypersegmented neutrophils, are not sensitive in an early deficiency. Nor are they specific to B12, since other situations bring them about.

In practical terms: a perfectly normal blood count does not rule out a B12 deficiency.

This matters all the more because nerve damage can appear before the blood abnormalities.

Symptoms of vitamin B12 deficiency

NICE and American Family Physician describe a broad picture.

Persistent fatigue and breathlessness on exertion, linked to anemia when it is present.
Tingling or numbness, most often in the feet and hands.
Problems with balance or walking.
Difficulty concentrating, memory problems or "brain fog"; sometimes low mood.
Sometimes a smooth, painful tongue, or vision problems.

None of these signs is specific. Fatigue accompanies many other situations, starting with a lack of iron. Concentration problems are found in prolonged stress or lack of sleep. For this reason, the test is read alongside the whole picture, and not in isolation.

One point deserves attention: prolonged neurological damage can become irreversible. The British guideline says treatment should not be delayed when strong clinical features conflict with the test result, in order to avoid neurological impairment.

Who is most at risk of vitamin B12 deficiency?

Certain situations clearly increase the risk. Universal screening is not recommended: American Family Physician suggests considering a test when there is at least one risk factor and one clinical feature, and NICE takes a similar approach.

Age

American Family Physician lists older age among the risk factors. Reduced stomach acid and atrophic gastritis, more frequent with age, make B12 from food harder to absorb.

Metformin

This medication, very widely prescribed in type 2 diabetes, hinders the absorption of B12. In a randomized trial cited by American Family Physician, low or borderline B12 was found in 19.1% of people on metformin against 9.5% on placebo. The American Diabetes Association Standards of Care suggest considering periodic B12 testing during long-term treatment, especially with anemia or peripheral neuropathy.

Long-term acid-reducing medication

Proton pump inhibitors (PPIs such as omeprazole), taken for reflux, reduce the stomach acid needed to release the B12 contained in food. A 2023 meta-analysis cited by American Family Physician linked PPI use to B12 deficiency, with an odds ratio of 1.42. NICE also lists H2 blockers.

A plant-based diet

Plants do not provide B12 in a dependable amount. People on a vegan diet need fortified foods or a supplement. Vegetarians who eat little dairy and few eggs are exposed as well, according to the Academy of Nutrition and Dietetics (2025).

Digestive diseases and surgery

Pernicious anemia, which is an autoimmune condition of the stomach, surgery on the stomach or intestine, celiac disease or Crohn's disease all disturb absorption.

Key point - With older age, metformin, long-term acid-reducing medication, a plant-based diet or digestive surgery, the risk of deficiency is higher. These elements change the reading of a result that sits in the uncertain zone.

Methylmalonic acid and homocysteine: the second-line tests

When the test leaves a doubt, two second-line tests exist. Ordering them is up to your doctor, depending on the context.

Methylmalonic acid (MMA) rises when B12 is lacking at the cell level. The British guideline presents it as the more specific of the two, and it is the test both American Family Physician and NICE point to after an indeterminate result. ABIM lists a reference range up to 0.40 µmol/L. MMA also rises with kidney disease and tends to be higher in older adults, according to the NIH.

Homocysteine (ABIM range: 5 to 15 µmol/L) also rises with a shortfall, but it goes up for other reasons as well, in particular a folate deficiency. It is therefore less specific.

NICE also uses active B12 (holotranscobalamin) as a possible first test; the UK cutoffs are below 25 pmol/L for confirmed deficiency and 25 to 70 pmol/L for indeterminate results. Not all laboratories offer it.

The BioSense method

The same result of 280 pg/mL (207 pmol/L) does not carry the same weight in a 35-year-old with no treatment and in a 70-year-old who has been on metformin for five years with tingling in the feet. BioSense sets each biomarker against your age, your treatments, your symptoms and your diet, then follows its trend from one test to the next.

*ChatGPT answers you. BioSense knows you.* *BioSense, your prevention companion.* BioSense does not provide a diagnosis and does not replace your doctor. It is a prevention companion that helps you understand, ask the right questions and act day to day.

Vitamin B12: three questions to ask your doctor

If your result sits in the uncertain zone, or if symptoms persist, these three questions help steer the visit:

My result sits in the zone where the test does not settle the question. Given my symptoms and my treatments, should we check methylmalonic acid?
Can any of my current medications hinder the absorption of vitamin B12?
If monitoring is needed, how often should this test be repeated and in which laboratory?

Mention on your own a diet low in animal products, or digestive surgery long ago: this often changes the reading of the result. Also mention any B12 supplement, as it can raise the level on the test.

Frequently asked questions

What is a normal B12 level?

There is no universal normal or ideal B12 level: each laboratory sets its own range, printed on your report. As an indication, US references list 200 to 800 pg/mL (148 to 590 pmol/L, ABIM) or 299 to 1,054 pg/mL (219 to 773 pmol/L, MedlinePlus). According to NICE, deficiency is unlikely above 350 pg/mL (258 pmol/L).

How much vitamin B12 do you need per day?

In the US, the Recommended Dietary Allowance is 2.4 mcg per day for adults, 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding. People on a vegan diet generally cannot reach it without fortified foods or a supplement; the choice is worth discussing with a health professional.

Do you need to fast for a vitamin B12 blood test?

Sometimes. MedlinePlus says you may need to fast for several hours before a vitamin B blood test. Follow your laboratory's instructions, especially if other tests are ordered on the same draw.

B12 in pg/mL, ng/L or pmol/L: how do you compare results?

Units of pg/mL and ng/L are equivalent, and 1 pg/mL corresponds to about 0.74 pmol/L. A result of 238 pg/mL therefore equals 238 ng/L, or about 176 pmol/L, in the zone where the test does not settle the question.

*This article is strictly informational and constitutes neither medical advice nor a diagnosis. For any question about your health, consult your doctor.*

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