What is the difference between active vitamin B12 and total vitamin B12?

There are several ways to measure your vitamin B12 level, but how can you be sure you are checking the right thing?

Around 11% of under 65’s in the UK are deficient in B12 [1]. This may not seem that many but that equates to over 6 million people being deficient in a nutrient that is vital to our health. But did you know there are two different kinds of vitamin B12? 

In this blog we discuss: 

What is vitamin B12?

Vitamin B12, also known as cobalamin, is an essential nutrient which we need to get from our food.  

Vitamin B12 has a role in several functions in the body, including: 

  • Keeping your body’s nerves and blood cells healthy 
  • Making DNA 
  • Metabolising fats and proteins 

Making sure you have enough B12 in your diet may also make all the difference in preventing conditions, such as dementia and anaemia. 

Without getting sufficient vitamin B12 from your food, the body cannot work optimally and can cause unwanted symptoms.  

Symptoms of a vitamin B12 deficiency include [2]: 

  • Extreme tiredness (fatigue)  
  • Feeling faint  
  • Headaches  
  • Pale skin 
  • Mood changes such as depression and/or irritability  
  • Noticeable heartbeats (palpitations)  
  • Pins and needles  
  • Mouth ulcers  
  • Memory problems  

If you are experiencing any of these symptoms, we recommend you speak to your GP, or test yourself

If you have a B12 deficiency, it is best to try and detect it early, as there are conditions and potential complications that can come alongside it if the deficiency is long-term.  

Potential complications of a B12 deficiency include: 

  • Heart conditions or heart failure  
  • Low mood  
  • Temporary infertility  
  • Chronic tiredness and low energy levels  

However, a deficiency in vitamin B12 can easily be detected through a blood test, and in most cases, can be easily reversed. 

vitamin b12 blood test product banner

But, with several ways to test vitamin B12 available, you may be wondering – which test checks for what, and how do I know if what I’m checking is the right thing?  

What are the different types of vitamin B12?

Once inside the body, there are two proteins that vitamin B12 can attach to, and each protein affects how the vitamin works. 

1. Haptocorrin

Scientists have not fully figured out the role of haptocorrin, but it may protect or transport vitamin B12 in our blood. When attached to this protein, vitamin B12 becomes inactive. 

2. Transcobalamin II (also known as active vitamin B12)

Transcobalamin II is the protein which transports vitamin B12 into our cells where the vitamin can be biologically active. When bound to this protein, vitamin B12 becomes active vitamin B12 [3]. 

What is total vitamin B12?

Total vitamin B12 refers to all the vitamin B12 in your blood. Total vitamin B12, therefore, includes the vitamin attached to both proteins haptocorrin and transcobalamin II [4]. 

Why should you measure active vitamin B12, not total vitamin 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.  

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 [5]. When attached to this protein, vitamin B12 cannot be taken in and used by your cells. 

Therefore, a normal total vitamin B12 level could mislead you into thinking that you have healthy vitamin B12 levels - even when your body cannot use most of it. And most likely cause a ‘false normal result’. So, in some cases testing total vitamin B12, could hide a deficiency.  

If you are pregnant or take the combined contraceptive pill, a B12 deficiency could be flagged via a total B12 blood test, when in fact these women could have sufficient amounts of B12 and not be deficient – creating a false result.  

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! 

At Medichecks, we only test your active vitamin B12 to get you the most accurate results. With our Vitamin B12 (Active) Blood Test, if your results are lower than they should be, one of our doctors may recommend appropriate lifestyle changes to get your vitamin B12 back into a healthy range.  

You may also find that you are recommended tests for pernicious anaemia, an autoimmune cause of low vitamin B12. But don’t worry, our doctors will let you know your next steps.  

People with active vitamin B12 that is at the low end of the normal range may still have a B12 deficiency. So, our doctors may recommend a serum MMA test to look for this. 

For more on B12, read our blog: is a vitamin B12 deficiency affecting your health?.



  1. Hunt, A., Harrington, D. and Robinson, S., 2014. Vitamin B12 deficiency. BMJ, 349(sep04 1), pp.g5226-g5226. 
  2. nhs.uk. 2022. Vitamin B12 or folate deficiency anaemia - Symptoms. [online] Available at: <https://www.nhs.uk/conditions/vitamin-b12-or-folate-deficiency-anaemia/symptoms/> [Accessed 11 July 2022]. 
  3. Human Nutrition, 12th Edition (2011). Edited by Catherine A. Geissler and Hilary J. Powers 
  4. Nice.org.uk. 2022. [online] Available at: <https://www.nice.org.uk/advice/mib40/resources/active-b12-assay-for-diagnosing-vitamin-b12-deficiency-pdf-63499159342789> [Accessed 11 July 2022]. 
  5. Nexo, E. and Hoffmann-Lücke, E., 2011. Holotranscobalamin, a marker of vitamin B-12 status: analytical aspects and clinical utility. The American Journal of Clinical Nutrition, 94(1), pp.359S-365S. 

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