Clarity Labs
Login
Find your panel
Vitamins & Minerals

Active B12 (Holotranscobalamin)

Active B12 is the small part of your vitamin B12 — only about a quarter — that your cells can actually take up and use. It shows whether B12 is really reaching your body more directly than the standard total-B12 test. A result in the expected zone is reassuring; a low one is common, often easy to put right, and read with the rest of your picture.

Targeted Testing
CategoryVitamins & Minerals
Reference rangeSee your report
Evidence○Targeted Testing
Last updated3 July 2026
On this page

What is active b12 (holotranscobalamin)?

Active B12, also called holotranscobalamin (or holoTC for short), measures the portion of vitamin B12 that is bound to a carrier protein called transcobalamin. This binding is what allows B12 to be taken up by cells throughout the body. Approximately one quarter of circulating B12 is in this holotranscobalamin-bound form; the rest is bound to another protein called haptocorrin and cannot be directly used by cells. By measuring only the bioavailable fraction, active B12 is considered the best direct measure of the B12 available to your body's tissues. The standard total serum B12 test measures all forms together — both the usable and non-usable fractions — so a normal total B12 result does not always mean your cells are getting enough B12 to function.

Why is it tested?

Active B12 is ordered when a GP wants a more precise read on your B12 than the standard total serum B12 test gives. It is most useful in two situations. The first is when a total B12 result lands in the indeterminate range — the grey zone where it is unclear whether you are deficient. The second is during pregnancy, because total B12 naturally dips in pregnancy even when there is no real deficiency. Because active B12 looks only at the usable (biologically available) fraction, it can pick up early B12 shortfall sooner than the total test. Its everyday use is limited by cost and how widely Australian labs offer it. The result is read alongside serum B12 and two "functional" markers — methylmalonic acid (MMA) and homocysteine — both of which rise in the blood when your tissues are running short of B12.

Reference range (Australia)

Decision thresholds: deficiency likely below 25; indeterminate 25–70; deficiency unlikely above 70 pmol/L

These are interpretive decision thresholds from Australian Prescriber (Bedz & Forsyth 2026), not a single 'normal range', and they are assay-dependent — use the interval printed on your own report. Within the indeterminate 25–70 pmol/L band, MMA and homocysteine can help clarify whether functional deficiency is present.

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

What your result means

If your level is low

A result below the deficiency-likely threshold suggests your cells may not be getting enough B12 to function well. Low active B12 can result from insufficient B12 in the diet (common in long-term vegan or vegetarian eating), reduced absorption from conditions affecting the stomach or gut (such as autoimmune gastritis or coeliac disease), or certain medicines taken long term. Your GP will look at the full picture — diet, symptoms, and related markers including MMA and homocysteine — to understand the cause and what to do. A low result on its own is not a diagnosis.

If your level is in range

A result in the indeterminate zone (between the deficiency-likely and deficiency-unlikely thresholds) is genuinely uncertain: it is not possible to say with confidence whether deficiency is present from active B12 alone. If your result falls here, your GP may add MMA or homocysteine to clarify whether B12 is reaching your tissues in sufficient amounts. A result clearly above the deficiency-unlikely threshold makes deficiency unlikely, though symptoms and risk factors are always part of the assessment.

If your level is high

A result above the deficiency-unlikely threshold generally means your cells have adequate access to B12. A very high active B12 may reflect recent supplementation or a diet rich in B12 from animal foods. Less commonly, certain liver or blood conditions can raise the level; in those cases the number is usually explained by the underlying condition. Your GP will consider your symptoms, diet, and any other markers alongside this result.

What can affect your result

The main driver of active B12 levels is how much B12 your body is absorbing and storing. Dietary intake matters: B12 is found naturally in animal-based foods including meat, fish, poultry, eggs, and dairy. Plant foods do not contain B12 naturally, so people following a long-term vegan or vegetarian diet can gradually deplete their stores unless they use fortified foods or supplements. Absorption is equally important. B12 from food is released by stomach acid and then bound to intrinsic factor (a protein the stomach makes) before being absorbed in the small intestine. Conditions that disrupt this process can lower active B12 even on a nutritious diet. Examples include autoimmune gastritis (where the immune system destroys the stomach cells that make intrinsic factor), coeliac disease, Crohn's disease, and previous bowel surgery. Several common medicines can also reduce B12 over time. The recognised ones are metformin (used for type 2 diabetes), proton pump inhibitors (strong stomach-acid reducers, taken for more than two years), and H2-receptor antagonists (another kind of acid-reducing medicine). Recent supplements or B12 injections, on the other hand, can lift active B12 and may hide an absorption problem underneath.

When to act

If your active B12 is in the deficiency-likely range, it is worth a prompt conversation with your GP. They will look at your diet, any medicines you take, and symptoms such as tiredness, numbness, tingling, or changes in mood or thinking. They may also check MMA and homocysteine to confirm how B12 function is tracking at the tissue level, and may look at total B12 and other markers to find the underlying cause. If your result is in the indeterminate zone and you have ongoing symptoms or risk factors, a GP review is still the right step — they can judge whether adding functional markers or a clinical assessment is warranted. If your result is clearly in the deficiency-unlikely range and you have no symptoms, follow-up is usually not needed, though your GP may 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

Australian Prescriber uses decision thresholds rather than a single 'normal range': below 25 pmol/L is considered deficiency likely; 25 to 70 pmol/L is indeterminate; above 70 pmol/L is deficiency unlikely. The range printed on your own report is the most relevant guide, as it reflects the specific assay your lab uses. Your GP reads your result alongside your symptoms and other B12 markers.

A low active B12 can suggest that your cells are not receiving enough B12, which may reflect insufficient intake, an absorption problem, or the effect of certain medicines. It is not a diagnosis on its own. Your GP will consider your diet, symptoms, related markers — homocysteine and methylmalonic acid (MMA) — and your medical history before deciding on next steps. This is one piece of the B12 puzzle.

The standard total serum B12 test measures all forms of B12 in your blood — both the fraction your cells can use and the fraction bound to a different carrier protein that cells cannot access. Active B12 (holotranscobalamin) measures only the usable fraction. This makes it a more direct indicator of whether B12 is actually available to your tissues, though it is not widely available as a routine first-line test in Australia.

Homocysteine and methylmalonic acid (MMA) are functional markers — they rise in the blood when tissues are not getting enough B12 to carry out their normal chemical reactions. If both are normal alongside a low active B12, deficiency at the tissue level is less likely. If they are raised, it suggests B12 function is genuinely impaired. Your GP may add one or both when an active B12 result falls in the indeterminate zone, or when the clinical picture is unclear.

Active B12 is most often ordered when a total B12 result falls in the indeterminate range, where it is not clear whether deficiency is present. It is also preferred in pregnancy, because total B12 naturally falls during pregnancy even without true deficiency — making the total test harder to interpret. In some Australian labs, active B12 is now available as a Medicare-rebated first-line test alongside total B12.

Test your active b12 (holotranscobalamin)

NATA-accredited testing. Results in 2–5 days. No GP referral required.

View Vitamin B12