Active Vitamin B12 (Holotranscobalamin)
The part of your vitamin B12 that is attached to its carrier protein and ready for your cells to use. It's often called 'active B12'.
AI-verified against cited sources. Awaiting expert reviewAlso called HoloTC, Holo-TCLOINC 72160-5
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What it is
Vitamin B12 travels in your blood on two proteins. Only the B12 on one of them, transcobalamin, can be taken up by your cells. Holotranscobalamin (holoTC, or 'active B12') is that ready-to-use portion, and it is usually a minor share of the total B12 in your blood. Because it falls quickly when new B12 stops arriving, studies find it a moderately better indicator of B12 status than total B12.
Why it matters
Your body needs B12 to make red blood cells, keep nerves healthy and make DNA. A shortage can cause tiredness, anemia, tingling or numbness, balance problems and trouble with memory or concentration. In the UK, NICE lists active B12 as an initial test alongside total B12, and as the preferred first test in pregnancy, when total B12 normally falls even without a true shortage.
If it’s low
In UK guidance (NICE NG239), a result below 25 pmol/L confirms B12 deficiency, and 25 to 70 pmol/L is a gray zone where a second test, such as methylmalonic acid, may be added. Low B12 is common in people who eat little animal food and in people who can't absorb B12 well. Your lab may use different cut-offs, so compare with the range on your report.
Common causes
- A diet with little or no animal food and no fortified food or B12 supplement
- Atrophic gastritis or pernicious anemia, which reduce B12 absorption
- Stomach or bowel surgery, including many weight-loss operations
- Celiac disease or another autoimmune condition
- Some medicines, including metformin, acid-reducing medicines and colchicine
- Recreational nitrous oxide use
What can help
- Ask your clinician whether your diet, medicines or past surgery could explain it
- Ask whether methylmalonic acid or homocysteine should be checked
- Ask what treatment, if any, is right for you; don't start or stop supplements on your own
If it’s high
A high result most often means you've recently had B12 from supplements, fortified products or injections. Reduced kidney function can also raise it. A high result is rarely a concern on its own, but a clinician may look further if supplements don't explain it, since high total B12 has been linked to liver and blood disorders.
Common causes
- Recent B12 supplements, multivitamins or injections
- Reduced kidney function
- Rarely, liver or blood disorders (seen with high total B12)
What can help
- Tell your clinician about every B12-containing product you use, including multivitamins and energy drinks
- Ask how a high result affects the way your other B12 tests are read
If this applies to you
Active B12 means a little more for some people. Open the one that fits you.
Vegan or vegetarian
Active B12 (holotranscobalamin) is the part of B12 ready for cells to use. A 2016 review of B12 in vegetarians found it a more useful marker than total B12, though birth control pills and kidney or liver disease can affect it. The Academy lists it among the tests for B12 status.
What can affect your result
- Recent B12 supplements, multivitamins and injections raise the result within days
- Kidney function: results rise as kidney function falls
- Pregnancy: holoTC stays steady while total B12 falls, which is why it is preferred then
- Assay and lab: cut-offs vary, and the 25 and 70 pmol/L thresholds are a guide
- Age, sex and transcobalamin genotype have small effects, and information on differences between ethnic groups is limited
- Recent vitamin B12 from supplements, multivitamins or injections raises the result without fully treating a shortage (NICE NG239). In one study, a daily 500 µg oral dose raised holoTC by about 50% within 3 days.
- Reduced kidney function can raise holoTC, because the carrier protein is small and is filtered by the kidneys.
- Recreational nitrous oxide use can cause B12 deficiency, and NICE advises using homocysteine or methylmalonic acid, not active B12, as the first test in that situation.
- Assay and lab: holoTC runs on several automated platforms (Abbott, Siemens, Roche, Beckman) that share a common reference calibrator, but local ranges and cut-offs can still differ.
Faster than total B12. It rises within days of taking B12 (about a 50% rise by day 3 in one study of a 500 µg daily dose) and falls quickly when new B12 stops arriving from food.
When to talk to a clinician
Talk with a clinician soon if your result is below 25 pmol/L, especially if you have numbness or tingling, balance problems, memory changes or marked fatigue. Tell your clinician promptly about any nerve symptoms, because UK guidance (NICE) says clinicians may not wait for test results before acting when anemia and nerve symptoms point to B12 deficiency.
Questions you could ask
- Would active B12 or total B12 be the better test for me?
- Do my supplements affect how this result should be read?
- Could my supplements or my kidney function explain this result?
- Should I change anything about how my B12 tests are interpreted?
- Should we check methylmalonic acid or homocysteine as well?
- Could my diet, medicines or past surgery explain a low result?
Questions people ask
What is the difference between active B12 and total B12?
Total B12 counts all the B12 in your blood. Active B12 counts only the part that cells can take up. In some studies it was a moderately better indicator of B12 status, which is why some clinicians use it or add it.
What does a result between 25 and 70 pmol/L mean?
It's a gray zone. UK guidance calls it indeterminate, which means B12 deficiency is possible but not confirmed. A clinician may check methylmalonic acid or homocysteine and consider your symptoms before deciding what to do.
Can my multivitamin change the result?
Yes. B12 from a multivitamin, a supplement or an injection can raise your active B12 within days and may hide a shortage. Tell your clinician what you take before testing.
Show the science and sources
Is a change real?
Active B12 naturally varies about 13% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 37% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Biological variation of holotranscobalamin and cobalamin in healthy individuals (Brokner M et al.).
For clinicians and researchers
HoloTC is B12 bound to transcobalamin (TC), which delivers B12 to cells through receptor-mediated uptake. It makes up about 5–20% of total TC, and TC carries only a minor part of circulating B12 (most of the rest is bound to haptocorrin). HoloTC runs on automated platforms from Abbott (since 2006), Siemens (2016), Roche (2017) and Beckman Coulter (2020), with calibrators traceable to one reference calibrator, which gives better harmonization than total B12 assays. NICE NG239 (2024) uses <25 pmol/L as confirmed deficiency, 25–70 pmol/L as indeterminate and >70 pmol/L as deficiency unlikely; a 2011 review proposed a consensus healthy interval of about 40–200 pmol/L but asked labs to confirm it locally, and no population-specific intervals have been set. HoloTC is moderately better than total B12 against methylmalonic acid and homocysteine in ROC analyses, but any cut-off trades sensitivity against specificity. Because TC is about 40 kDa, it is filtered by the kidney and holoTC rises as kidney function falls, although it is affected less than methylmalonic acid. Unlike total B12, holoTC is not lowered by the fall in haptocorrin during pregnancy. High-dose B12 and nitrous oxide exposure can mislead any serum B12 measure, and in nitrous oxide-related deficiency, homocysteine or methylmalonic acid should lead.
LOINC 72160-5 (primary). Specimen: serum.
Sources
- Vitamin B12 deficiency in over 16s: diagnosis and management (NG239), National Institute for Health and Care Excellence (NICE) (2024)
- The application and interpretation of laboratory biomarkers for the evaluation of vitamin B12 status, Annals of Clinical Biochemistry (2025)
- Holotranscobalamin, a marker of vitamin B-12 status: analytical aspects and clinical utility, American Journal of Clinical Nutrition (2011)
- Personalising laboratory medicine in the 'real world': assessing clinical utility, by clinical indication, of serum total B12 and Active-B12, Annals of Clinical Biochemistry (2021)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.