1,25-dihydroxyvitamin D (calcitriol)
This test measures 1,25-dihydroxyvitamin D, the biologically active form of vitamin D. It is used to investigate calcium imbalance, kidney disease and...
Vitamin B12 (Active)
78.00
The Active Vitamin B12 test measures holotranscobalamin, the biologically active form of vitamin B12. It helps identify early deficiency before total B12 levels fall.
Turnaround time
2 days
Biomarkers count
1
Same-Day Appointments
UKAS Accredited Labs
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Under 18? Patients under 18 can only be seen at GB Medlabs and Clinilabs, Monday to Friday only.
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Details about Vitamin B12 (Active)
What is the Active Vitamin B12 test?
The Active Vitamin B12 test measures holotranscobalamin (holoTC), the fraction of vitamin B12 that is directly available to cells. Unlike total B12 testing, which measures all circulating B12, this test focuses on the portion that the body can actually use.
Only a small percentage of total vitamin B12 is biologically active. As a result, total B12 levels can appear normal even when cells are functionally deficient, making active B12 a more sensitive marker in early or borderline cases.
Clinically, the Active B12 test is used to assess suspected deficiency, investigate neurological symptoms, and clarify inconclusive total B12 results.
Why do I need a test for Active Vitamin B12?
Active Vitamin B12 deficiency can occur before total B12 levels drop below the reference range. This makes the test particularly valuable when symptoms are present but standard B12 results are borderline or normal.
It is especially useful for people with absorption problems, older adults, vegans, or those with neurological or cognitive symptoms. Early identification allows treatment before irreversible nerve damage occurs.
You may need an Active Vitamin B12 test if you experience:
- Fatigue or low energy with normal total B12
- Brain fog, memory issues, or poor concentration
- Tingling, numbness, or burning sensations
- Balance or coordination problems
- Unexplained anaemia
- Digestive or absorption disorders
Benefits of the Active Vitamin B12 test
- Detects early B12 deficiency
Identifies functional deficiency before total B12 becomes low. - More sensitive than total B12 testing
Focuses on the biologically usable fraction. - Supports neurological investigations
Useful when nerve-related symptoms are present. - Clarifies borderline results
Helps interpret inconclusive total B12 findings. - Improves treatment accuracy
Guides appropriate supplementation or injections. - Supports cognitive health assessment
Low active B12 is linked to brain fog and memory changes. - Useful in absorption disorders
Relevant for pernicious anaemia and gut conditions. - Reduces misdiagnosis risk
Prevents missed deficiency due to misleading total B12 levels.
Step-by-step – How we offer this test
Option 1: Clinic-based blood test (UK-wide)
- Choose your nearest UK clinic
- Attend your appointment
- A clinician draws a blood sample
- Results are processed and reported
Option 2: Home or hotel visit (+£60, UK-wide)
- A qualified phlebotomist comes to your location
- Blood sample taken in your home or hotel
- No need to travel
- Results handled the same as clinic testing
Your results explained
Low Active Vitamin B12 levels
Low active B12 indicates functional vitamin B12 deficiency, meaning cells are not receiving enough B12 even if total levels appear normal.
UK reference range (low):
- < 35 pmol/L
Low levels are associated with fatigue, neurological symptoms, anaemia, and cognitive impairment.
Normal Active Vitamin B12 levels
Normal levels suggest sufficient biologically active B12 is available for cellular function, including nerve health and red blood cell production.
UK reference range (normal):
- 35 – 150 pmol/L
Symptoms are unlikely to be caused by B12 deficiency within this range.
High Active Vitamin B12 levels
High active B12 levels are most commonly due to supplementation or injections and are usually not harmful.
UK reference range (high):
- > 150 pmol/L
Unexplained high levels may occasionally warrant further investigation.
How It Works
Simple steps to get your results
1
Book Online
Choose your test online and book in seconds. Select your preferred clinic location or home visit option.
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Visit or Test at Home
Attend one of our UK or London clinics, arrange a home nurse visit, or use a finger-prick kit where available.
3
Get Your Results
Your sample is analysed by accredited UK laboratories, with secure results delivered directly to you.
Take control of your health with London Blood Tests
WhatsApp UsHow to Prepare for Your Test
Follow these guidelines for accurate results
Sample Timing
Where possible, attend your blood test between 7am and 12pm. Please wait until any short-term illness or infection has fully resolved before testing. Avoid intense exercise for 24–48 hours beforehand, as this can affect certain markers.
Fasting & Hydration
Fasting is not always required, but some tests may recommend it. If fasting is advised, avoid food for 8–12 hours before your appointment and drink water only. Stay well hydrated, as this helps with sample collection and accuracy.
Medications & Supplements
Continue prescribed medications unless advised otherwise by your clinician. Avoid vitamin, mineral, or biotin supplements for at least 24–48 hours before your test, as these can interfere with results. If you are unsure about any medication or supplement, please let us know before your appointment.
Hormones & Menstrual Cycle
For hormone-related tests, timing within your menstrual cycle may be important. If relevant, follow any specific guidance provided on your test page.
If you have any questions or special circumstances, our team is happy to advise before your appointment.
Frequently Asked Questions
Total B12 measures all circulating B12, while active B12 measures the portion available for cellular use.
It is more sensitive for early or functional deficiency, especially when total B12 is borderline.
Fasting is not usually required, but supplements may be paused if advised.
Yes, this is one of the main reasons for testing active B12.
People with neurological symptoms, fatigue, absorption issues, or borderline B12 results.
Yes, supplements and injections can raise active B12 levels.
Usually when symptoms are present or during treatment monitoring.
Yes, it is particularly useful for people on plant-based diets.
Yes, functional deficiency can impair red blood cell production.
High levels from supplementation are usually safe but unexplained elevation should be reviewed.
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