Holotranscobalamin (holoTC, active B12) is a direct blood measurement of the fraction of vitamin B12 bound to transcobalamin, the only form cells can take up. Practitioners use it when total B12 is borderline or symptoms do not fit a normal total, especially with plant-based diets, metformin or acid-suppressing medication. Recent oral B12 lifts it within days, so it reflects recent intake more than stores.
| Test type | Measured in a blood sample |
|---|---|
| Measurements required | Holotranscobalamin (active B12) only |
| Sample and collection | Blood draw at a collection centre |
| Preparation | No fasting is needed for this test. |
| Turnaround | 1 to 3 business days |
| Available through | Vitamins & Trace Elements. Available in: Active Vitamin B12. |
| Pricing | Price on sign-up |
| Results | Returned in the Vively practitioner platform, beside your client's other markers |
What are the Holotranscobalamin (active B12) reference and optimal ranges?
| Typical lab reference | Vively optimal | |
|---|---|---|
| Adults | Above 35 pmol/L | 100 pmol/L or above |
Labs differ, and some report an indeterminate band near the lower limit, so read the result against the range and comments on the client's report (RCPA guidance). The optimal range is a target for review, not a diagnostic cut-off.
Check a client's Holotranscobalamin (active B12) result
Enter the value from your client's lab report to see where it sits against the typical lab range and the Vively optimal range.
A guide only, not a diagnosis. One result is a starting point: read it with symptoms, history and a repeat test. Labs differ, so check the result against the range on your client's report. Nothing you enter is stored or sent.
How do you arrange Holotranscobalamin (active B12) testing through Vively?
- Get verified. Practitioners with a recognised qualification or membership (ATMS, NHAA, ANTA, Dietitians Australia, NSA, CMA or Ahpra registration) can request free practitioner access.
- Choose a panel. Log in to the practitioner platform and pick a panel that includes Holotranscobalamin (active B12), or build your own. Add-on pricing shows when you sign up.
- Client collects. Your client collects at a partner collection centre, with their request on their phone or printed.
- Review results. Results come back to you in the practitioner platform, beside your client's other markers, ready for the follow-up consult.
Which Vively panels include Holotranscobalamin (active B12)?
Log in to the practitioner platform to view a panel in full, or build and save your own from 100+ tests and panels.
- Active Vitamin B121 markers · NutrientsPrice on sign-upHolotranscobalamin (active B12)Log in to view this panel
Why does the optimal range sit there?
Our 100 pmol/L floor sits well above the 35 pmol/L lab cut-off, clear of the indeterminate zone just above it.
HoloTC carries the 10 to 30% of circulating B12 that cells can use, and falls before total B12 when supply drops (Nexo and Hoffmann-Lücke, American Journal of Clinical Nutrition, 2011).
Below about 35 pmol/L, deficiency is likely. Just above it, the British Society for Haematology guideline (Devalia et al., 2014) advises methylmalonic acid or a repeat test where symptoms suggest deficiency.
A client above 100 pmol/L is unlikely to be short of active B12, though no outcome trial sits behind 100 pmol/L specifically.
What should Holotranscobalamin (active B12) be read with?
- LowLow holoTC, total B12 in range
Possible early or functional deficiency that total B12 misses; check MCV and discuss MMA with the GP.
- ContextNormal holoTC, low total B12
Often seen in pregnancy or on the oral contraceptive pill, where haptocorrin falls and true deficiency is less likely.
- LowLow holoTC with low ferritin
Raises the question of atrophic gastritis or coeliac disease affecting absorption of both.
- LowLow holoTC, low RBC folate, high MCV
A combined deficiency pattern that needs GP review before any folic acid is started.
- HighHigh holoTC with reduced eGFR
Kidney impairment raises holoTC, so read high or normal results against kidney function.
What can affect a Holotranscobalamin (active B12) result?
- Recent oral B12 lifts holoTC within 24 to 48 hours, faster than total B12
- Reduced kidney function raises holoTC and can mask low supply
- Nitrous oxide inactivates B12 inside cells, so holoTC can read normal while tissue deficiency develops; MMA or homocysteine are more useful there
- Pregnancy lowers holoTC less than total B12, but pregnancy-specific cut-offs are not well established
- Assays and indeterminate bands differ between labs, so compare repeat results from the same lab
When should you refer the client to their GP?
- HoloTC below the lab range, or indeterminate with numbness, tingling, unsteady gait or memory change
- Low holoTC with high MCV or low haemoglobin
- Any history of recreational nitrous oxide use with neurological symptoms, whatever the result
- Suspected malabsorption: coeliac disease, atrophic gastritis, or previous gastric or bariatric surgery
How should a client prepare for a Holotranscobalamin (active B12) test?
Copy or print this to send to your client.
Preparing for your Holotranscobalamin (active B12) test
- No fasting is needed for this test.
- Tell your practitioner about any B12 supplements and when you last took one, as active B12 rises within a day or two of a dose.
- Take your pathology request to any of 4,000+ collection centres. Bring it printed or on your phone.
- Keep taking prescribed medicines as usual unless your doctor tells you otherwise.
- Results usually take 1 to 3 business days and come to your practitioner. Book your results appointment for after that.
How do practitioners use Holotranscobalamin (active B12) with clients?
Use holoTC to settle borderline total B12 results and to screen plant-based clients, in whom holoTC tends to fall first. Because it responds within days to supplements, it reflects recent intake more than stores in a client who has just started B12.
Holotranscobalamin (active B12) FAQs
Can a naturopath, nutritionist or dietitian order holotranscobalamin through Vively?
Yes. Add it to any client panel on the Vively practitioner platform. It sits in Vitamins & Trace Elements and is also in Active Vitamin B12. Add-on pricing shows when you sign up. No account yet? Request free access.
Is holotranscobalamin better than total B12?
It is more sensitive to early deficiency, though neither test settles every case alone. HoloTC falls first when intake or absorption drops, while total B12 can stay in range for months. Total B12 is better validated and more familiar to GPs. Ordering both, then MMA through the GP for indeterminate results, gives the clearest answer.
Can holoTC show whether a client absorbs oral B12?
In principle, yes. The holoTC absorption test compares a baseline sample with one taken after a short, standardised course of oral B12, and a clear rise suggests absorption is working. It is a specific protocol, so set it up with the client's GP rather than improvising, and read it alongside the clinical picture.
Does kidney function affect holotranscobalamin?
Yes. Reduced kidney function raises holoTC and can mask low B12 supply, so a high or normal result needs reading against kidney function. Check the eGFR and creatinine on the same report before treating a normal holoTC as reassuring in a client with kidney impairment, and involve the client's GP where kidney function is reduced.
Why can holoTC look normal in a client with a history of nitrous oxide use?
Nitrous oxide inactivates B12 inside cells, so holoTC can read normal while tissue deficiency develops. MMA or homocysteine, arranged through the client's GP, are more useful in that situation. Any history of recreational nitrous oxide use with neurological symptoms warrants GP review whatever the holoTC result shows.
References
- Royal College of Pathologists of Australasia. RCPA Manual: Active B12 (holotranscobalamin).
- Nexo E, Hoffmann-Lücke E. Holotranscobalamin, a marker of vitamin B12 status: analytical aspects and clinical utility. American Journal of Clinical Nutrition. 2011. Source
- Devalia V, Hamilton MS, Molloy AM. Guidelines for the diagnosis and treatment of cobalamin and folate disorders. British Journal of Haematology. 2014.
- National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). 2024. Source