Active vitamin B12 vs. total vitamin B12 - what's the difference and which should I test?
Learn the difference between active and total vitamin B12, and which you should test to determine your levels.
The best way to detect a vitamin B12 deficiency is with a blood test. But did you know there are different kinds of vitamin B12? In this article we explain which test checks for what, and how to know which is best for you.
We’ll discuss:
- What is vitamin B12?
- What are the different types of vitamin B12?
- What is total B12?
- Why should you measure active instead of total B12?
What is vitamin B12?
Vitamin B12 is a water-soluble nutrient that is used in many of the body's processes.
B12 helps your body with:
- DNA production
- The normal functioning of the nervous system
- Cell repair
- Metabolism of fats and proteins
It’s an interesting molecule, as it’s the only vitamin that contains a trace metal – cobalt. That’s where B12 gets it’s chemical name – cobalamin – from. Although we get our sources of vitamin B12 from animal products, it’s actually synthesised by microorganisms.
Dietary sources of vitamin B12
Vitamin B12 is an essential nutrient, which means our body can’t synthesis it.
Animal sources of vitamin B12 include:
- Meat
- Fish
- Milk
- Eggs
But if you are vegan or follow a mostly plant-based diet, don’t worry, you can still get vitamin B12 from food.
Plant-based sources of vitamin B12 include:
- Yeast extract (found in Marmite,Vegemite, and nutritional yeast)
- Unsweetened fortified plant milk
- Fortified breakfast cereals
What are the different types of vitamin B12?
Once vitamin B12 enters the bloodstream, it binds to carrier proteins. There are two proteins that B12 can attach to, and each protein affects how the vitamin works and whether it can be used by your cells.
1. Haptocorrin (inactive vitamin B12)
Most vitamin B12 binds to haptocorrin. It’s thought this helps to transport the B12 in the blood. However, B12 that is bound to haptocorrin can’t be used by most body tissues, so we say it is inactive. This accounts for around 70-90% of your vitamin B12.
2. Transcobalamin II (active vitamin B12)
A smaller portion of vitamin B12 binds to transcobalamin II. This complex is known as active B12, and the vitamin is delivered directly into cells, where it can be used [2]. This accounts for between 10-30% of your vitamin B12 [3].
What is total B12?
So, now we know about inactive and active vitamin B12, what is total vitamin B12?
You may have already guessed it: total vitamin B12 refers to all the vitamin B12 in your blood.
A total vitamin B12 blood test measures:
- B12 bound to haptocorrin (inactive), and
- B12 bound to transcobalamin II (active) [3]
Because it includes inactive B12, a total B12 result doesn't always reflect how much B12 your body can actually use.
Why should you measure active instead of total B12?
As total vitamin B12 combines both types of vitamins, you’d assume that this would be the best thing to measure – but not quite.
Total B12 can mask a deficiency
About 85-90% of the vitamin B12 in your blood is not available for your body to use because it is strongly attached to the protein haptocorrin. When attached to this protein, vitamin B12 cannot be taken in and used by your cells.
This can lead to a “false normal” result, where:
- You could have a normal total B12 result
- But still have insufficient levels of B12
A normal total vitamin B12 level could mislead you into thinking that you have a healthy amount – even when your body can’t use most of it.
Active B12 reflects what your body can actually use
On the other hand, active vitamin B12 (the quantity bound to transcobalamin II) makes up only 10-15% of vitamin B12 in your blood [3], but it is the most important kind. This is the biologically active form of vitamin B12 and therefore takes part in all your body’s essential biological reactions – so this is the one to measure!
The active form of vitamin B12:
- Enters cells
- Supports DNA synthesis
- Supports nerve function
- Prevents deficiency symptoms
At Medichecks, we only test your active vitamin B12 to give you the most accurate results.
References
- Hunt A, Harrington D, Robinson S. Vitamin B12 deficiency. BMJ. 2014;349: g5226. doi:10.1136/bmj.g5226
- Prevalence | Background information | Anaemia - B12 and folate deficiency | CKS | NICE. [cited 17 Jul 2026].
- Nexo E, Hoffmann-Lücke E. Holotranscobalamin, a marker of vitamin B-12 status: analytical aspects and clinical utility12345. Am J Clin Nutr. 2011;94: 359S-365S. doi:10.3945/ajcn.111.013458
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