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Vitamins & Minerals

Vitamin B12

Vitamin B12 keeps your nerves working and helps your body make red blood cells and DNA. It comes entirely from food (mainly meat, fish, eggs and dairy) or supplements — your body cannot make it. A blood test gives a useful first look, but it measures B12 in your bloodstream, not how much is reaching your cells. A result near the lower end sometimes needs a follow-up test to know for certain whether you are running short.

Standard Care
CategoryVitamins & Minerals
Reference rangeSee your report
Evidence✓Standard Care
Last updated3 July 2026
On this page
5.2–6.3%

Among Australians aged over 50, an estimated 5.2 to 6.3% have vitamin B12 deficiency.

What is vitamin b12?

Vitamin B12, also called cobalamin, is an essential nutrient found mainly in animal-based foods. Your body needs it to form and repair red blood cells, nerve cells and brain cells, and to build DNA. Because your body cannot make B12 itself, you rely on diet or supplements. The liver can hold a store that lasts a long time, which is why deficiency often develops slowly and quietly. The serum B12 test — serum meaning the liquid part of your blood — measures the total amount of B12 circulating in your bloodstream.

Why is it tested?

Serum B12 is often the first test ordered when something looks off. That might be symptoms like tiredness or nerve changes (tingling, numbness), or a flag on a Full Blood Count — a routine blood test that checks your red cells — such as unusually large red blood cells or anaemia. It is also relevant for people at higher risk: those following a vegan or vegetarian diet, people with conditions that affect gut absorption (such as coeliac disease or Crohn's disease, or who have had bowel surgery), and people taking certain medicines long term. Total serum B12 is typically the first-line test.

Reference range (Australia)

Decision thresholds: deficiency likely below 133; indeterminate 133–258; deficiency unlikely above 258 pmol/L

These are interpretive decision thresholds (Australian Prescriber), not a "normal range", and they are assay-dependent — use the interval printed on your own report. Within the indeterminate 133–258 band, serum B12 can miss functional deficiency, so Active B12, methylmalonic acid or homocysteine help clarify.

Reference intervals vary between laboratories. Always use the range printed on your own report.

What your result means

If your level is low

A low B12 suggests a deficiency, and the usual next step is to look at the underlying cause rather than act on the number alone. Common reasons include low dietary intake (particularly long-term vegan or vegetarian diets), reduced gut absorption from bowel conditions, and pernicious anaemia — an autoimmune condition where the stomach stops making the protein needed to absorb B12 from food. Long-term use of some medicines can also be a factor. A result in the indeterminate band near the lower limit sits in a known grey zone, because the standard serum test cannot show how much B12 is actually reaching your cells. Your GP may add other markers — Active B12 (holotranscobalamin), methylmalonic acid or homocysteine — to clarify the picture.

If your level is in range

A result above the deficiency thresholds usually means deficiency is unlikely, and any symptoms you have may have another cause worth exploring. Because the serum test reflects blood rather than tissue levels, a normal number does not completely rule out a functional shortfall in people with strong symptoms or clear risk factors. Worth raising with your GP if you have ongoing concerns.

If your level is high

A raised B12 is uncommon and usually not monitored on its own. It often reflects recent supplementation or a B12-rich diet. Less commonly, a high total B12 can occur alongside certain conditions — such as inflammation, liver disease or some blood disorders — where the elevated number does not reflect how much B12 is reaching the tissues. A high result is best understood in context, and your GP can help make sense of it alongside the rest of your picture.

What can affect your result

Diet is the main driver. Animal foods — meat, fish, eggs and dairy — are the natural sources, and B12-fortified foods contribute for those who eat fewer of them. B12 does not occur naturally in plant foods, so a long-term vegan diet with no fortified foods or supplements will gradually deplete stores. Absorption depends on a healthy stomach and bowel. Pernicious anaemia (an autoimmune condition that stops the stomach making the absorption protein it needs) can cause deficiency even on a good diet. Bowel conditions like coeliac disease and Crohn's disease have the same effect. Some medicines used long term can quietly lower B12 levels: metformin (a common diabetes medicine), proton pump inhibitors (stomach-acid reducers used for more than two years), and H2-receptor antagonists (another class of acid-reducing medicine) are recognised examples. Recent supplementation can raise the blood level and may mask an underlying tissue deficiency.

When to act

If your B12 is low, or sits in the indeterminate grey zone and you have symptoms such as tiredness, numbness, tingling or changes in thinking, it is worth a conversation with your GP. They can look at your diet, medicines and gut health, and may add Active B12, methylmalonic acid or homocysteine to clarify whether a true deficiency is present. If your level is clearly in the deficiency-unlikely range and you have no symptoms, follow-up is usually not needed — though your GP may choose to keep an eye on things if you have ongoing risk factors. This page is general information and is not a substitute for advice from your GP.

Frequently asked questions

In Australia, B12 is reported in pmol/L and interpreted using decision thresholds rather than a single "normal range". One Australian clinical guide describes deficiency as likely below 133 pmol/L, indeterminate between 133 and 258 pmol/L, and deficiency unlikely above 258 pmol/L. Your report will show the interval your lab uses, and your GP reads it alongside your symptoms and risk factors.

The standard serum test measures B12 in your blood, but cannot show how much is actually inside your tissues and being used by your cells. That is why a result near the lower limit is treated with caution — it sits in a grey zone where the number alone is not enough to be certain. Follow-up tests that measure what is happening at the tissue level can give a clearer answer.

Active B12 (holotranscobalamin) — the fraction of B12 that is actually bound in a form your cells can take up — is considered the best direct measure of usable B12 in the body. Methylmalonic acid (MMA) and homocysteine are functional markers: they rise in the blood when tissues are not getting enough B12 to do their job. If both MMA and homocysteine are normal, deficiency is unlikely. Your GP may add one or more of these when a serum B12 result falls in the indeterminate range.

B12 does not occur naturally in plant foods, so people following a long-term vegan diet rely on fortified foods or supplements to maintain their levels. Vegetarians who eat eggs and dairy usually get more, but can still run low over time. If you follow a plant-based diet, it is worth raising with your GP at your next visit.

Yes. Metformin (a common diabetes medicine) and proton pump inhibitors used for more than two years are recognised causes of lower B12. H2-receptor antagonists — another type of acid-reducing medicine — are also a factor. If you take any of these long term, mention it to your GP when you discuss your result.

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