The most common way to check your B12 levels is a simple blood test that measures the amount of vitamin B12 circulating in your bloodstream. You can get this test through your doctor with a standard blood draw, or you can use an at-home finger-prick kit that you mail to a lab. Either way, results typically come back within a few days and are reported in picograms per milliliter (pg/mL).
The Standard B12 Blood Test
A serum B12 test is the starting point. Your doctor can order it as part of routine bloodwork or in response to specific symptoms. The test requires a blood draw from your arm, and most labs process the results quickly.
You may need to fast for 6 to 8 hours beforehand to prevent food from skewing results. If you take B12 supplements, skip your dose before the test. Supplements can temporarily inflate your blood levels and mask an actual deficiency, so timing matters.
What Your Results Mean
Labs vary slightly in how they define normal ranges, but most use these general thresholds:
- Below 200 pg/mL: Most laboratories consider this deficient.
- 200 to 300 pg/mL: This gray zone is often called “insufficiency.” About 12.5% of adults 19 and older fall into this range or below.
- Above 300 pg/mL: Generally considered normal.
The tricky part is that gray zone between roughly 150 and 400 pg/mL. Your levels might technically be “in range” while your body still isn’t getting enough B12 to function well. If your results land there, your doctor will likely order a follow-up test to get a clearer picture.
Follow-Up Tests That Confirm a Deficiency
A standard B12 blood test tells you how much B12 is in your blood, but not how well your body is actually using it. Two additional markers help clarify that.
Methylmalonic Acid (MMA)
B12 plays a key role in breaking down a substance called methylmalonic acid. When your body doesn’t have enough B12, MMA builds up because it isn’t being processed efficiently. An MMA level above 0.271 micromol/L suggests a true B12 deficiency, even if your serum B12 number looked borderline. This is considered one of the most reliable ways to confirm that your cells are actually short on B12.
Homocysteine
Homocysteine is an amino acid that B12 helps convert into other useful compounds. When B12 is low, homocysteine accumulates. A level above 15 micromol/L points toward B12 deficiency. However, homocysteine can also rise due to low folate levels, so it’s less specific than MMA on its own. Doctors often use both tests together to build a complete picture.
Testing for Pernicious Anemia
If your B12 is clearly low and your doctor suspects your body can’t absorb it properly, they may test for pernicious anemia. This autoimmune condition destroys a protein in your stomach called intrinsic factor, which you need to absorb B12 from food. The intrinsic factor antibody test checks for this. It’s highly specific, meaning a positive result is a reliable indicator, though it can occasionally miss cases. ELISA-based versions of the test tend to offer the best combination of sensitivity and accuracy.
At-Home B12 Test Kits
If you’d rather skip the doctor’s office for initial screening, at-home test kits are widely available online. These kits include materials for a finger-prick blood sample that you collect yourself and mail back to a lab. Some test B12 alone, while others bundle it with other vitamins or a broader micronutrient panel.
The convenience is real, but there are limits. At-home kits measure serum B12 only. They won’t include MMA or homocysteine testing, which means they can’t confirm a deficiency on their own. If your at-home result comes back low or borderline, your doctor will almost certainly want to retest with a standard lab draw and potentially order those additional markers before making any diagnosis.
Symptoms That Warrant Testing
Many people check their B12 out of curiosity, but certain symptoms make testing especially worthwhile. B12 deficiency affects the nervous system directly, sometimes before anemia ever shows up on a blood count. Pins and needles in your hands or feet, difficulty with balance or coordination, memory problems, and vision changes are all neurological signs linked to low B12. Fatigue and weakness are common but less specific. If you’re experiencing numbness or tingling alongside persistent tiredness, B12 testing should be a priority rather than an afterthought.
Certain groups are more likely to run low: adults over 50 (who absorb less B12 from food), people on a vegan or strict vegetarian diet, anyone taking long-term acid-reducing medications, and people with digestive conditions affecting the small intestine. If you fall into one of these categories, periodic screening is reasonable even without symptoms.
How to Request the Right Tests
If you’re asking your doctor to check your B12, request a serum B12 test as the baseline. If the result is below 400 pg/mL and you have symptoms, ask specifically for an MMA test as well. Many doctors won’t automatically order MMA unless the serum B12 is clearly low, but experts recommend it for anyone in that borderline 150 to 399 pg/mL range. Adding homocysteine to the panel costs little extra and gives your provider another data point. The combination of all three tests is the most reliable way to determine whether you’re truly deficient or just on the lower end of normal.