How to Test for Vitamin B12 Deficiency - Check These 4 Tests

How to Test for Vitamin B12 Deficiency (Order These 4 Lab Tests)

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Doctors Miss Vitamin B12 Deficiency All of the Time

Vitamin B12 might be one of the most important vitamins in the body because every single cell in your body needs it. 

And if you are deficient in this important vitamin you may start to experience problems in energy production, nerve cell growth, and other areas

Low vitamin B12 levels may cause fatigue, nerve problems, hair loss, weight gain, heart problems, and more. 

Because your Vitamin b12 levels are so important we are going to spend some time talking about how to test for a deficiency in this important marker. 

The good news is that testing is actually not that difficult. 

The bad news is that most doctors order the wrong tests or simply don’t know how to interpret them correctly…

4 Tests to Check for Vitamin B12 Deficiency

So what are you supposed to do if you think that you have a problem with low vitamin B12 levels?

Well, the first step is to TEST!

Testing for vitamin B12 will not only tell you if there is enough B12 in your bloodstream but it will also help you to know if your body is actually using that B12 (but only if you order the right tests). 

And while you may think it’s as easy as just checking your serum B12 levels, you’d be wrong and you would miss out on 3 other important tests!

If you want to accurately diagnose vitamin B12 deficiency then THESE are the tests that you need to get: 

#1. Serum B12 Level

The first test you should be aware of is known as the serum B12 level. 

The serum is just a fancy way of describing what is in your bloodstream. 

So serum B12 refers to the amount of B12 that is floating around in your bloodstream and in your body. 

But there’s one big problem…

Just because this B12 is floating around in your bloodstream does NOT mean that your body is actually using it. 

And this is a concept I want to make sure you understand. 

Just because something is in your blood does not mean that your body can use it. 

In fact, in the case of B12, it’s often the case that even though serum B12 levels are “normal” the body isn’t actually able to utilize the B12 floating around because it’s in the wrong form or because of genetic mutations. 

And doctors know this!

This is first-year medical school-type information (1) that all doctors should be taught and it’s why the next test exists (MMA). 

We will talk more about MMA in a second but I want you to understand here that if your doctor insists upon ordering and only looking at your serum B12 level then you know that he/she does not truly understand how B12 works in the body. 

In a similar vein, if your doctor suggests that you are taking too much vitamin B12 because your serum B12 is too high then you know that they definitely don’t know what they are talking about. 

Even though your serum B12 level may increase while you supplement with B12 it does NOT mean that your body is actually using it. 

Low-quality B12 supplements like cyanocobalamin will register in your bloodstream but they may not be activated or methylated due to MTHFR genetic mutations. (2) 

#2. MMA (Methylmalonic acid)

The next test that you can order is known as methylmalonic acid or MMA for short. 

What is methylmalonic acid and what does it have to do with vitamin B12 deficiency?

Without getting into the weeds, vitamin B12 is required as a cofactor for certain enzymatic pathways in the body. 

If your vitamin B12 levels fall to a certain point then your body will have a hard time running these pathways at an optimal rate. 

So as B12 levels fall it slows down the metabolism of these pathways which results in a build-up of methylmalonic acid. 

And methylmalonic acid is a MUCH better marker of vitamin B12 deficiency because it actually tells you if your body can utilize the B12 floating around in your bloodstream. 

Doctors know this, or at least they should because they were taught this in their first year of medical school. 

This is why I recommend that you run away from doctors who are dead set on using the serum B12 level to diagnose vitamin B12 deficiency. 

But back to the MMA test for a second. 

Even though checking your methylmalonic acid level is a much more accurate way to test for vitamin B12 deficiency it still has its cons. 

The biggest is the cost of the test. 

It’s actually more expensive to get your blood drawn and tested for MMA than it is to simply buy a vitamin B12 supplement and take it. 

Because of this, it’s often recommended to simply skip the test and just go straight to supplementing if you have the clinical symptoms of vitamin B12 deficiency. 

I’m not a huge fan of the MMA test and rarely ordered it for my patients for that very reason. 

But, if you really want to accurately measure B12 metabolism in your body then the MMA test is the way to go. 

#3. MCV (Mean Corpuscular volume)

The next two tests that we are going to talk about are not specific to vitamin B12 deficiency but they can be used to help paint a broad picture of your overall health and to give you information about vitamin B12 metabolism. 

The first test that you will most likely have in your possession, even as you read this, is known as the MCV. 

MCV is a part of the complete blood count or CBC. 

The CBC is the test that your doctor orders to check your red blood cells, and white blood cells and checks to make sure you are not anemic. 

It’s incredibly cheap and it’s ordered as part of your yearly checkup so there is a very high chance that you have this test result on hand. 

The MCV stands for mean corpuscular volume and it gives you information about the size of your red blood cells. 

What you need to know is that as your vitamin B12 levels fall it triggers a change in the size of your red blood cells such that they start to grow bigger (3). 

And bigger is NOT better, at least not in this case. 

You want your red blood cells to be a nice even and regular size in order to facilitate the carrying of oxygen to your tissues. 

As they grow bigger they start to lose some of this functionality which can cause serious problems. 

An MCV greater than 90 or 92 can be used as an early marker of vitamin B12 deficiency. 

And because this test is so cheap, and because very few things influence the MCV, it’s a good idea to take a look at this value if you think you might have a B12 deficiency. 

#4. Homocysteine level

The next test worth considering is known as the homocysteine level. 

Much like the MCV, homocysteine can give you information about whether or not vitamin B12 is being used in the body. 

Here’s why:

Homocysteine can be broken down and metabolized into another protein known as cysteine. 

But in order for this process to occur your body needs vitamin B12 to make the reaction function optimally. 

So if your vitamin B12 level is low then you will see a build-up of homocysteine because it cannot be broken down. 

Therefore, a high homocysteine level may be an early indicator of vitamin B12 deficiency (4). 

It can also be a warning sign that your body cannot metabolize vitamin B12 even if your serum B12 level is “normal”. 

Homocysteine is not often ordered but having a high homocysteine level can increase your risk of heart disease so it’s a very good idea to check where you are at the next time you get your blood drawn. 

By using both homocysteine and the MCV you can really get a good idea if your body is able to metabolize and use vitamin B12. 

The Case for Supplementing Without Testing

Let’s talk about one more final and very important point…

Is it even necessary to test for B12 deficiency before supplementing?

If you ask me, I don’t think it’s 100% necessary to go through all of these tests BEFORE you supplement with Vitamin B12. 


Well, first of all, the tests themselves cost more money than purchasing a pre-methylated B vitamin. 

And secondly, there’s no single test that is 100% accurate in diagnosing the condition. 

No matter how thorough you are, laboratory testing is simply not always accurate. 

It’s the best thing that we have, but each test has its pros and cons. 

And even if all tests show that you are “good”, it’s still possible that supplementing is still the right choice. 

When you factor in the harmless side effect profile of Vitamin B12 supplements, it makes it a no-brainer. 

Having said that, I still would recommend getting the lab tests listed above (with the exception of MMA) if it is convenient, cheap, and not a big hassle. 

The information provided by these tests, while not necessarily 100% accurate, can still provide you with a story about your overall health. 

For instance, these lab tests, when combined with others, may paint a picture of multiple nutrient deficiencies, a problem with your diet, and so on. 

So while the information may not be necessary for supplementing with B12 by itself, it may still be useful when evaluating your health as a whole. 

Final Thoughts

Do you suspect that you have a problem with vitamin B12 levels?

Do you have any of the symptoms of vitamin B12 deficiency?

Have you had any of the 4 tests listed above checked?

If so, leave your questions or comments below!





these 4 tests are required to diagnose vitamin b12 deficiency

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About Dr. Westin Childs

Hey! I'm Westin Childs D.O. (former Osteopathic Physician). I don't practice medicine anymore and instead specialize in helping people like YOU who have thyroid problems, hormone imbalances, and weight loss resistance. I love to write and share what I've learned over the years. I also happen to formulate the best supplements on the market (well, at least in my opinion!) and I'm proud to say that over 80,000+ people have used them over the last 7 years. You can read more about my own personal health journey and why I am so passionate about what I do.

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49 thoughts on “How to Test for Vitamin B12 Deficiency (Order These 4 Lab Tests)”

  1. Hi Dr Westin,
    Just saw your video on 6 stages of hashimotos . I think I may be on stay 6 how would I know for sure ? What test do your recommend for me find out if my thyroid still working. Would natural supplements work for me .. I also suffer from RA 10!years Hypothyroidism 15 years

  2. Hi, Dr. Childs.
    I’ve read the article about B12 vitamin deficiency. I’m positively sure that I’ve such a problem, because I’m a bariatric patient. I’ve done B12 vitamin investigation, but the result is:”The value of B12 is lower than the investigation kit can expose”. Now I’m taking shots with B12, but my family doctor doesn’t know the amount of B12 is optimal for me. Can you recommend me what can I do in this situation? Tks in advance.

  3. Question. If the body can’t process the b-12 will the homocysteine levels ever come down with supplementation? Mine are elevated. Thank you for a reply.

        • Hi Rachael,

          That’s my general recommendation. It’s often safer and more effective to just use premethylated B’s which is why I include them in all of my supplements.

          • Hi, Dr. Childs. What if I am slow COMT and cannot easily tolerate methylated B-12? Elevated MCV of 97. Homocysteine ok at 7. Will hydroxy do the trick or do I absolutely need methylated B-12?
            thanks, Mary

  4. I have a B12 of >1500
    I do not take supplements
    Homocysteine is normal
    MMA is normal
    MCV is normal
    I have hashimotos and a prior hx of aggressive angiomyxoma
    How do I find out if it’s really a B12 deficiency?

  5. I’ve taken normal b vitamins and had a low reading and switched to a different complex with methylated b’s and my levels are still low. MCV is 93. Would b12 shots be recommended? I’ve heard you need weekly and I’ve also heard daily. What are your thoughts? I’d like to be informed before I see my NP.

    • Hi Chelsea,

      B12 shots would be a good idea! The frequency is typically one shot per week depending on the dose.

  6. I have had a MMA test that shows Im deficient Im at 1.55 umol. All my other B test didn’t show any problems. So from what I’ve read that means i have the B12 but my body isnt using it. Does that mean
    i should go to shots? I already take a multi-vitamin which should give me like 1000 per cent of all the b vitamins, and still got these results. I am suffering from perennial neuropathy and foot drop. I have had all the tests to make sure its not ALS MS etc, is there any other tests to show the my body isn’t actively using B12?

  7. Could you talk a bit about the “Intrinsic Factor” and it’s role in the body’s utilization of B12? If you can’t assimilate it from your food due to a lack of Intrinsic Factor, can you assimilate it from B12 Shots?

    • Hi Mara,

      Intrinsic factor is required for absorption in the gut but doesn’t play a role outside of it. So you wouldn’t need to worry about it if you are taking your B12 via injections/shots.

  8. Hello Doctor. I suspect I am low on b12 but my doctor won’t test for the reasons you mentioned above. I want to start supplementing but I always break out horribly when I start b12. Any ideas as to why? And have you heard of low b12 causing dizziness and vertigo? Thank you doctor. What I’ve learned from you is so greatly appreciated.

    • Hi Diana,

      It’s probably due to using the wrong type of B12. Try a methylated version of B12 as that should do the trick!

  9. I have all the symptoms of B12 deficiency and had some blood tests which they said were normal. I had my large bowel and i believe a little of my small bowel in 1994. I have ileostomy bag. Now I am having severe problems inside my mouth and tongue. I am still convinced it is lack of B12 but how can i tell all the experts they are wrong?

    • Hi David,

      You don’t really need them on board in order to get something like a B12 shot so your time is probably better spent trying to get those than convincing your doctor to order it for you.

  10. Hello Dr. Childs!
    Thank you for your interesting article! I thank you in advance for answering my questions! 🙂

    Many years ago, I was diagnosed with macrocytic anemia & told I would need to take b12 injections from that point on. A long time passed, and a a year or two ago, my current doctor, seeing that my b12 levels were high, took me off of injections. My b12 levels of b12 have gradually decreases. It is still normal but my MCV is slightly out of range/high. I have all the symptoms of low b12 though. My Doctor keeps poo-pooing my suggestions that I still have anemia & dismissing them.

    I just had bloodwork at a pay as you go lab done to test MMA & am waiting for the results to come back. If they are out of range as expected, I figure I will do a Homocysteine test & then take both of the lab results back to my doctor to basically say No, I’m not just whining about nothing or having a mental issue. My question is, I stopped taking any B supplements for a few weeks prior to the MMA. Should I do this for the Homocysteine test too? I hope that is not daft question! My mcv is always between 92 & 99. Or has been for the past 6 years or so.
    Thanks so much!

    • Hi Elinor,

      It’s often best to just supplement with the B12 if you believe that you are deficient. Testing is fraught with inaccuracies and it’s almost impossible to overdose on B12, so it’s usually a waste of time/money to test.

  11. Hi Dr. Westin,
    I’m pretty certain I’m B12 deficient, both my sister and mother have PA. I’ve had testing which says I’m deficient but GP says I’m within range. I have read that if you are deficient then oral supplementation isn’t effective, is this true? I have supplemented with B complex with folate which increased my levels but my tiredness just won’t go away and now I’m experiencing Afib. What would you advise?

    Sue G

    • Hi Susan,

      Oftentimes oral B12 isn’t as effective as sublingual B12 or B12 shots, especially in the setting of pernicious anemia. I have personally found that B12 shots are more effective than any other form for most people.

  12. Hi Doctor
    My MCV is 92.5 and constantly high. My B12 seems normal. My homocysteine was elevated last year but lowered a little bit and now within “normal” limits. I have not tested MMA . Do you think these are signs of deficiency or is there another reason MCV can be high? I suffer from Hashi but also a neurological disease called small fiber (lots of nerve/muscle issues). I believe I may have another neurological autoimmune that may have SFN as a symptom as my issues are severe. Not convinced it’s just Hashi or SFN unfortunately. Trying to look at deficiencies. Thnx.

  13. Dear Doctor,
    I am a 78 year old vegan woman. I have a new doctor who refuses to order the MMA test. I test high for B12 and she seems concerned about the level. In the recent past I have had monthly shots of B12 (prescribed by my former doctor).These make my recent doctor uncomfortable. I experience at least six of the effects of low B12. I am now taking a methylated 2500 B12 and feel so much better after 2 weeks on this. She wants me to take 1000 and didn’t suggest the methylated.
    Three questions please:
    Can you die from or become seriously ill from high B12 levels?
    Can you take B12 supplements for life safely?
    Will the results of the tests you recommend be affected by the supplements I’m now taking? Should I stop them a while before the tests?
    I am sincerely concerned.

    • Hi Sharon,

      I’ve never seen a case report indicating any harm in using B12 regardless of dose 🙂 I’m sure there is a toxic dose of B12 that can be reached, but it seems like it’s insanely high and really nothing to worry about. Your serum B12 is meaningless for the reasons listed in this article. It doesn’t actually tell you how much B12 is being used by your cells.

  14. Thank you for this article. What should one do if one cannot metabolize B12 properly (has consistently high homocysteine levels even with methylated B12 supplementation)?

    • Hi Stef,

      There are many other potential causes of high homocysteine including diet, lifestyle (alcohol/smoking), poor thyroid function, and other autoimmune diseases.

  15. Dr. Childs,
    I had to take hydroxocobalimin shots because the other shots were making me ache all over. My Doctor passed away and I would like to know what oral B12 I can take to help me since I have Thyroid problems. I tried one brand of oral hydroxocobalimin and saw no difference
    What do you suggest?

  16. Dr. Childs,

    Can you help us understand why we are deficient in b12, how to eat it naturally to increase it and I learned from a course that you can check under your tongue and if big lumps, one is deficient? Think it is a TCM diagnoses. Also, would eating a little more dirt from garden veggies help increase b12 or buying soil organism supplements?

    Can you please advise/opinion?

    Many thanks for your insight,
    barbara CN

  17. Hi Dr. Childs,

    I had Graves Disease 36 years ago, treated with 2 doses of RAI a year apart. On Synthroid since. Been to numerous doctors & hospitals as their go to test is TSH and mine is always 0.01 (give or take) no matter what dose I’m on (200 mcg down to 88 mcg), so they constantly think I’m hyper even though symptoms are hypo. Anyway, I have been taking B12 shots for last 6 years. My most recent serum B12 test showed 390 (range 200-1100). They offered no other test so I advised my Alternative Therapy doctor I was going to add a B complex vitamin and a spray methycobalamin B12. I can only get cyanocobalamin shots.

    Also, of note, my cholesterol numbers go up every time they lower my synthroid dose. So, not only do they want to drop my Synthroid down because of TSH, but now they want to put me on cholesterol medication. I don’t know what tests I need anymore. I have been to Johns Hopkins, St. Joseph, GBMC and numerous doctors offices. I don’t know where to go anymore. Any suggestions would be greatly appreciated.

    Thank you, Nancy

  18. Hi, I have SNF. My B12 is 943, MMA is 71 with lab range at 8.7-318. Homocysteine 10 and MCV is 89.4. I current take 1000iu Vit D but plan change that to methylated. Since my MMA is so low how do I interpret this? Am I processing Vitamin D?

  19. Hi. Thank you for this post. I wanted to ask you about Holotranscobalamin (HoloTC), active B12.

    I’ve been reading that some think this is the best B12 marker. What do you think of this? Also, I can’t find any where to get the blood test. Thanks,

    • Hi Kevin,

      There are some studies that suggest that holotranscobalamin is perhaps more accurate than other measures but it’s not widely available and most likely not necessary. At the end of the day, it’s always just cheaper to take a B12 supplement if you think you are low. There’s virtually no downside and the cost of taking the supplement is usually much cheaper than the test.

  20. Hello Dr. Westin Childs,

    My daughter has B12 deficiency, but she said that this vitamin causes acne.
    She took pills from GNC.
    Unfortunately, she ceases to take it.
    Is other recommendation of B12 that you can make? It is important cause she has depression too. Thank you very much for helping!

    • Hi Ioana,

      It’s rare but possible that B12 could cause acne. The best thing to do would be to make sure you have a quality skin care routine to use with the B12.

  21. HI: I have an underactive thyroid and am taking Synthroid 0.137mg. It has been several years now ( can’t remember the year I was diagnosed). I have decreased energy and I want to sleep all the time. I have gained a considerable amount of weight (about 60 lbs. in all over the years) and have quite a bit of hair loss, and I suffer from depression for which I have medication. I have High Blood Pressure, IBS, and Acid Reflux and a Hiatal Hernia. I have a pinched Nerve in my shoulder from Arthritis in my Neck and Disc Compression, in a lot of back pain On a daily basis. How should I take my Acid Reducer…..I usually take it as soon as I get up and wait for 1/2 hr. before I eat. Then I take the Blood Pressure Pills, Thyroid medication and depression medication all together after I have eaten. I had Blood Work done (a CBC) and my Dr. told me that I should reduce the amount of Vit. B12 I was taking! When I read your comment about this I was shocked. What kind of Specialist should I try to get my Dr. to refer me to. Here in Canada we are required to have a referral to all specialists by our Family Dr.

    • Hi Kim,

      It looks like your comment was cut short. If you can come back and edit the comment or place another I’d be happy to reply.

  22. Hi there!
    I am taking iron & injecting B12 shots after falling into severe anaemia symptoms after Covid. My doctor approves of the iron, but not the shots.

    (Despite my history of veggie then vegan diet (life long), hypothyroidism, mould toxicity, heavy bleeding, & 20 years of heavy drinking.)

    The only blood test I’ve had prior to the shots (whilst I was taking regular B12 pills) was the MCV. It has been over 90 since 2021 (I can’t check further back.)

    Do you think this will be enough to persuade my GP that I’m doing the right thing? Obviously my serum test is high now, & was never tested before (!?!)

    And should I increase my iron now I’m supplementing B12?

    Many thanks!

    • Hi Anita,

      I guess I would ask why it matters if your PCP approves or not. B12 shots are quite safe and serum B12 levels are not accurate (as described in this article) so their value should be taken with a grain of salt.


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