Summary
Active B12 (holotranscobalamin) measures only the fraction of vitamin B12 that is bound to transcobalamin and actively delivered to cells. Because it represents the truly bioavailable pool, it detects functional deficiency earlier than Total B12 — before anaemia or neurological damage develops.
Total B12 is dominated by the haptocorrin-bound fraction, which is an inactive storage form that cells cannot use. This means total B12 can appear normal while functional deficiency is already causing cellular problems. Active B12 cuts through this by measuring only what the body can actually use.nnActive B12 is particularly useful in people with borderline total B12, neurological symptoms despite normal total B12, or at high risk of deficiency (vegans, older adults, metformin users, those with gut absorption issues).
What It Is
Vitamin B12 circulates in the bloodstream bound to two transport proteins. Approximately 70–80% is bound to haptocorrin — an inactive storage carrier from which cells cannot directly take B12. The remaining 20–30% is bound to transcobalamin II — the active delivery protein that transports B12 directly to cells. This active, transcobalamin-bound fraction is called holotranscobalamin or Active B12.nnBecause Active B12 represents only the immediately available, cell-deliverable pool, it falls earlier in the development of deficiency than total B12. It is therefore a more sensitive first indicator that B12 status is becoming inadequate, often detectable weeks before total B12 drops below the normal range.
Functions
Cellular B12 delivery marker
Measures the biologically available B12 fraction — the only component that cells can actively take up and utilise.
Neurological health indicator
Low Active B12 is the earliest detectable sign of B12 depletion before neurological symptoms or anaemia develop.
DNA synthesis support
Active B12 is the fraction that enters cells to support DNA synthesis, methionine production, and red blood cell maturation.
Homocysteine metabolism
Reflects the B12 available for homocysteine-to-methionine conversion — a key cardiovascular protective function.
Reference Ranges
Serum Active B12 (Holotranscobalamin)
Measured in pmol/L| Status | Range (pmol/L) | What it means |
|---|---|---|
| Deficient | < 25 | Active B12 depletion — cellular deficiency likely. Supplementation and investigation of cause required. |
| Borderline | 25–35 | Sub-optimal Active B12 — monitor and consider supplementation, particularly with symptoms present. |
| Adequate | 35–165 | Sufficient bioavailable B12 for normal cellular function. |
| High | > 165 | Elevated Active B12 — most commonly from supplementation. Generally not clinically concerning. |
Active B12 cut-off values vary slightly between laboratories and assay manufacturers. Interpret alongside Total B12, folate, and clinical symptoms for the most complete assessment.
Symptoms of Imbalance
Active B12 deficiency produces the same clinical spectrum as Total B12 deficiency but is detectable at an earlier, pre-symptomatic stage.
- Fatigue, weakness, and low energy
- Tingling or numbness in the hands and feet
- Difficulty with balance and coordination
- Memory impairment and cognitive slowing
- Low mood and depression
- Pale or yellowish skin tone
- Sore or inflamed tongue (glossitis)
- No symptoms from elevated Active B12 when due to supplementation — excess is excreted
- Very high Active B12 without supplement use warrants clinical investigation
Causes of Imbalance
- Vegan or strict vegetarian diet
- Pernicious anaemia (autoimmune intrinsic factor deficiency)
- Older age (reduced gastric acid and intrinsic factor)
- Metformin use
- Long-term proton pump inhibitor or antacid use
- Malabsorption (coeliac disease, Crohn's, post-bariatric surgery)
- Nitrous oxide exposure (inactivates B12)
- B12 supplementation (most common cause — harmless)
- Liver disease releasing stored B12
- Myeloproliferative disorders
FAQs
Total B12 is dominated by the inactive haptocorrin-bound fraction (70–80%), which cells cannot use. This means total B12 can appear normal while the body is actually running short of usable B12. Active B12 measures only the transcobalamin-bound fraction — the part that is actually delivered to cells. It detects functional deficiency earlier and more accurately.
Yes, this is the key diagnostic scenario where Active B12 testing adds value. When Total B12 is in the borderline or low-normal range, Active B12 can reveal functional deficiency that would otherwise be missed. This is particularly common in older adults and in those with a high haptocorrin-bound fraction that inflates total B12.
Active B12 is especially useful in: vegans and vegetarians, people over 60, those taking metformin or proton pump inhibitors, individuals with neurological symptoms despite borderline total B12, and anyone with fatigue or cognitive symptoms where standard B12 appears normal.
No fasting is required. You can eat and drink normally beforehand. Take any B12 supplements as usual — this is a test of your current nutritional status, and supplement doses will be reflected in the result, which is expected and informative.
Active B12 typically starts to rise within 2–4 weeks of starting supplementation. A repeat test at 3 months gives a clear picture of whether the deficiency has been adequately corrected. Neurological symptoms, if present, may take longer to resolve.
References
- Herrmann W, Obeid R. Cobalamin deficiency. Subcell Biochem. 2012;56:301–322. View source
- Nexo E, Hoffmann-Lücke E. Holotranscobalamin, a marker of vitamin B12 status. Clin Chem Lab Med. 2011;49(7):1141–1149. View source
- Sobczynska-Malefora A, et al. An audit of holotranscobalamin as a marker of B12 deficiency. J Clin Pathol. 2014;67(4):362–365. View source