Vitamin B12 is vital for good health. Your body needs steady levels of this nutrient to make enough red blood cells and keep your nervous system working.
If you eat a balanced diet, you’re probably getting enough B12. It’s found in animal products such as dairy products, eggs, fish, and chicken, as well as fortified cereals and nutritional yeast.
But there are reasons why the B12 in your body may dip below normal. A simple blood test can show whether your levels are healthy, low, or somewhere in between.
Do I Need a Vitamin B12 Blood Test?
“The most common reason someone would need to have their vitamin B12 levels checked is to evaluate for vitamin B12 deficiency,” says Mike Ren, MD, assistant professor of family and community medicine at Baylor College of Medicine.
Some reasons your body may not be able to get enough B12 from food include:
- Anemia
- Gastritis
- Crohn’s disease
- Celiac disease
- Alcohol use disorder
- Sjögren’s syndrome
- Weight loss surgery, or surgery that removes part or all of your stomach
- A vegan or strict vegetarian diet (as B12 is only found in animal products)
- Medications, such as metformin and drugs used to treat gastroesophageal reflux disease (GERD), indigestion, and stomach ulcers
- Pregnancy
Your doctor could also order a B12 test based on your symptoms.
Signs of low vitamin B12 include:
- Fatigue
- Nausea
- Throwing up
- Diarrhea
- Losing weight without trying
- A sore tongue
- A yellow tinge to your skin
- Fuzzy thinking
- Numbness or tingling in your hands and feet
- Vision changes
- Trouble walking or speaking
- Feeling easily irritated
- Confusion
- Depression
Many conditions share these same symptoms. A blood test that checks your B12 levels can rule out or confirm that a vitamin deficiency is the cause.
What Conditions Can a Vitamin B12 Blood Test Help Diagnose?
A low test result means you’re not getting or absorbing enough B12 from food. A high result means that your body isn’t flushing out extra B12 like it should. Either way, your doctor will want to know the cause so they can prescribe a treatment that can help.
Vitamin B12 deficiency
If left untreated, vitamin B deficiency can harm your brain and nervous system, leading to issues such as:
- Incontinence (not being able to control your pee or poop)
- Nerve issues, such as numbness, weakness, or nerve pain
- Erectile dysfunction (not being able to get an erect penis)
- Paralysis (not being able to move your body)
- Memory loss
- Paranoia (feeling like others are trying to hurt you)
If you don’t have symptoms but are at risk of low B12 levels, your doctor might also want you to have regular tests.
Can a vitamin B12 test diagnose anemia?
Pernicious anemia is a common reason for low B12. It’s an autoimmune condition that happens when your body attacks your stomach lining and nerve cells by mistake. That means you can’t absorb enough vitamin B12 from food.
If your doctor thinks you have pernicious anemia, they’ll order a B12 test. They’ll probably also order other lab tests. For instance, they can look for high methylmalonic acid (MMA) levels in your blood to confirm B12 deficiency.
Can a vitamin B12 test be used for weight loss?
You might see B12 products for sale that promise to help you lose weight. That claim isn’t backed by science. A few early studies have found a link between low B12 levels and obesity, but much more research needs to be done to confirm this link.
If you’re looking for new ways to get to a healthy weight, talk to your doctor. They can suggest simple changes to your daily routine, talk to you about treatment options, and offer other tips that can help.
Can a vitamin B12 test diagnose liver disease?
It’s very rare to have high levels of B12. Because it’s water-soluble (dissolves in water), you naturally pee out any excess. But some conditions, such as liver disease, cause your body to retain this vitamin. If your test results show higher than normal levels, your doctor will probably order more tests to check your liver health.
Can a vitamin B12 test diagnose leukemia?
If your doctor suspects you have leukemia, they may do a B12 test before ordering a more complex test, such as a bone marrow biopsy. Very low B12 levels can change your bone marrow cells just like leukemia and lymphoma. A B12 test can show if your symptoms are due to pernicious anemia rather than cancer.
What Does the Test Involve?
Before your visit, make sure your doctor knows about all the medications and supplements you take. They may ask you to stop them on the day of your test. You may also need to stop eating or drinking for six to eight hours before your appointment.
At the lab, you might feel a prick or sting as a very thin needle is put into one of your veins. A small sample of your blood will be collected for analysis.
Your doctor may also order other blood tests for you to get at the same time.
Should you use a home kit to test your B12 levels?
You may see do-it-yourself (DIY) B12 test kits for sale online, but there are many reasons to skip these, says Ren. For instance:
The results may not be right. “Not all home test kits are FDA-approved and calibrated for accuracy,” says Ren. “Some may give false highs or lows.”
If you don’t follow the steps correctly, you could get false results.
The scope is limited. Many of these home kits only measure total B12, and not other markers in your blood, such as methylmalonic acid or homocysteine, which are better signs of deficiency, Ren says.
You won’t know why you got a certain result. Without a doctor’s input, you won’t know if a low B12 test result is because of your diet, medications that you take, or a health issue.
You won’t get any guidance. “Self-testing doesn’t provide treatment advice, like dosage for supplements or need for injections,” says Ren.
You could also risk your privacy, since some home kit vendors may sell or misuse your health data, Ren says.
Understanding the Results
The normal range for vitamin B12 can vary slightly depending on the lab.
Generally:
- A normal level of vitamin B12 in your bloodstream is 160-950 picograms per milliliter (pg/mL).
- Between 200 and 300 pg/mL is considered borderline. Your doctor may do more tests.
- Below 200 pg/mL is low. More testing is needed.
Talk to your doctor about what your specific results mean.
Moving Forward
If your vitamin B12 test shows that your levels are healthy, you don’t need to do anything but keep eating a balanced diet.
But if you have low vitamin B12 levels, you’ll likely need to get more into your system with your doctor’s help. They could prescribe B12 as a supplement you take by mouth, a nasal gel or spray, or a shot.
It’s important to follow your doctor’s treatment plan. Vitamin B12 deficiency can lead to severe problems with your brain function, nervous system, and other aspects of your health.
Sometimes, you only need extra B12 for a short time. In other cases, you might need to stay on it for years.
If you’re older than 75, you’re at increased risk for low levels of vitamin B12. Talk to your doctor about whether your diet offers enough B12 or if taking a supplement would help.
Takeaways
A B12 test can tell how much vitamin B12 you have in your body. Having too much or not enough can cause serious symptoms and may be a sign of another condition. A vitamin B deficiency is often easily treated with supplements or B12 shots, but your doctor may order other blood tests first to get more details about your health.
Vitamin B12 Blood Test FAQs
What does a high vitamin B12 indicate?
It’s rare to have too much B12 in your blood. It’s a water-soluble vitamin, which means your body will flush out any excess. A high result on a blood test could signal another health issue — for instance, liver or kidney disease. Your doctor will run more tests to try to find the cause.
Does B12 affect the thyroid?
Your body needs B vitamins to control thyroid function. Many people who have hypothyroidism (an underactive thyroid) are also low in vitamin B12. There may be many reasons for this, including the foods you eat or other health issues, such as autoimmune diseases, that you live with.
Can I check for vitamin B12 deficiency at home?
There are several reasons why testing your B12 level at home might not be a good idea. Your results may be wrong or hard to understand. If your B12 levels are low or high, you won’t know the next steps to take. If you have questions about your B12 levels, talk to your doctor.
What is the cost of a vitamin B12 test?
There’s no one price for a B12 test. How much you’ll pay depends on several factors, including whether your doctor orders it and for what reason, if you have health insurance, the type of plan you have, where you live, and even the lab where you get the test.