Vitamin B12 test results: B12, active B12, MMA and homocysteine
A vitamin B12 test measures how much B12 is in your blood. Vitamin B12 helps keep your nerves and blood cells healthy, and it is often measured together with folate. (MedlinePlus, B12 level) (MedlinePlus, vitamin B test) NICE says a single blood test can be enough to support a diagnosis of B12 deficiency, but some people need more tests. (NICE NG239, 2024)
Who may be offered a B12 test
The test is most often done when other blood tests suggest a type of anemia called megaloblastic anemia, or when there are nerve symptoms such as numbness or tingling in the arms and legs, weakness or loss of balance. (MedlinePlus, B12 level) NICE advises offering a test to people who have at least 1 common symptom or sign and at least 1 common risk factor, and using judgment when there are symptoms but no risk factor. (NICE NG239, 2024)
- Common symptoms and signs (NICE): anemia or large red cells on a blood count, memory or concentration problems, some eyesight problems, glossitis (an inflamed tongue), balance problems and falls, pins and needles or numbness, and unexplained tiredness. (NICE NG239, 2024)
- Older age: MedlinePlus lists being older than 75 as a risk factor, and notes that older people often cannot absorb enough B12 from food. (MedlinePlus, MMA test) (MedlinePlus, homocysteine)
- Diet: a diet that leaves out or is low in animal foods, such as a vegan diet, without fortified foods or B12 products; also food allergies, or difficulty getting or preparing food. (NICE NG239, 2024)
- Stomach or bowel surgery: many weight-loss operations and removal of part of the stomach or bowel. (NICE NG239, 2024) (MedlinePlus, MMA test)
- Some medicines: NICE lists several, including a common diabetes medicine and medicines that reduce stomach acid. (NICE NG239, 2024)
- Health conditions: celiac disease, Crohn disease, a type of stomach inflammation (atrophic or autoimmune gastritis) and other autoimmune conditions such as thyroid disease, Sjögren disease or type 1 diabetes; a family history of B12 deficiency; and alcohol use disorder. (NICE NG239, 2024) (MedlinePlus, MMA test)
NICE also says the absence of anemia does not rule out B12 deficiency, and that B12 deficiency can be linked with mental health symptoms. (NICE NG239, 2024) Low B12 is one of the causes checked when a memory test is low: see memory screening (Mini-Cog and MoCA).
What each test shows
| Test | What it shows |
|---|---|
| Total B12 (serum cobalamin) | How much vitamin B12 is in your blood. It is the usual first test. (MedlinePlus, B12 level) (NICE NG239, 2024) |
| Active B12 (holotranscobalamin) | Serum holotranscobalamin, another way to measure B12. NICE allows it as the first test instead of total B12, and uses it first in pregnancy. (NICE NG239, 2024) |
| MMA (methylmalonic acid) | A substance that builds up when the body does not have enough B12. A high level can be a sign of B12 deficiency, and it is used to confirm one. (MedlinePlus, MMA test) (MedlinePlus, B12 level) |
| Homocysteine | An amino acid the body breaks down using vitamins B12, B6 and folate, so it can be high when any of them is low. (MedlinePlus, homocysteine) (NICE NG239, 2024) |
The test is a blood sample from a vein in your arm. MedlinePlus says not to eat or drink for about 6 to 8 hours before it, and that some medicines can affect the result; do not stop any medicine without talking to your provider. (MedlinePlus, B12 level) NICE says blood for the test should be taken before any B12 treatment starts, and that you may be asked whether you already use a product containing B12. (NICE NG239, 2024) See fasting before a blood test.
Typical ranges
MedlinePlus lists a normal total B12 of 299 to 1,054 pg/mL (219 to 773 pmol/L), and says values below 299 pg/mL are a possible sign of deficiency. Ranges vary slightly between labs, and some labs use different units or samples. (MedlinePlus, B12 level) Experts are not sure what a normal level is for some B vitamins, and labs differ, so the range on your report is the one that applies. (MedlinePlus, vitamin B test)
NICE uses three bands to judge how likely deficiency is, unless the local lab has its own validated cut-offs. (NICE NG239, 2024) One ng/L of B12 is the same number as one pg/mL.
| Total B12 | Active B12 | What NICE says it means |
|---|---|---|
| Less than 180 ng/L (133 pmol/L) | Less than 25 pmol/L | Confirmed B12 deficiency |
| 180 to 350 ng/L (133 to 258 pmol/L) | 25 to 70 pmol/L | Indeterminate: deficiency is possible |
| More than 350 ng/L (258 pmol/L) | More than 70 pmol/L | Deficiency is unlikely |
Source for the bands: NICE NG239 table 1. (NICE NG239, 2024) For MMA and homocysteine, NICE says to use the lab's own reference range, and to remember that other things, such as low folate, can also raise homocysteine. (NICE NG239, 2024)
- Things that can change a B12 result: using a product that contains B12 can raise the level without fully correcting a deficiency, and the combined birth control pill can lower total B12 without causing a deficiency. (NICE NG239, 2024) NICE also notes that people of Black ethnicity may have a higher reference range for B12 than people of White or Asian ethnicity. (NICE NG239, 2024)
- Homocysteine: levels can rise with age, are usually higher in males, and rise in females after menopause. They can also be high with conditions such as chronic kidney disease or an underactive thyroid. A high level does not always mean a condition that needs treatment. (MedlinePlus, homocysteine)
What low or high results can mean
- Low B12: causes include too little B12 in the diet (rare, except with a strict vegetarian diet), conditions that stop the gut absorbing it such as celiac disease and Crohn disease, a lack of intrinsic factor (a protein needed to absorb B12), an overactive thyroid, and pregnancy. (MedlinePlus, B12 level)
- High MMA: may mean B12 deficiency. A lower-than-normal MMA is not common and is not considered a health problem. (MedlinePlus, MMA test)
- High B12: uncommon, because extra B12 is usually removed in the urine. Conditions that can raise it include liver disease (see liver function test results) and some blood disorders. (MedlinePlus, B12 level)
For red cell size and anemia on a blood count, see complete blood count results.
Borderline results and follow-up tests
- Indeterminate result with symptoms: NICE suggests considering an MMA test. (NICE NG239, 2024) MedlinePlus also says a low B12 should be confirmed with MMA, and a high MMA points to a true deficiency. (MedlinePlus, B12 level)
- Indeterminate result without symptoms: NICE says to seek medical help if symptoms or signs develop. (NICE NG239, 2024)
- Result says deficiency is unlikely, but symptoms continue: NICE says to look for other causes and consider repeating the first test after 3 to 6 months. (NICE NG239, 2024)
- Finding the cause: NICE suggests an anti-intrinsic factor antibody blood test when autoimmune gastritis is suspected, noting that a negative result does not rule it out, and a blood test for celiac disease if the cause is still unknown. (NICE NG239, 2024)
Where they differ:
- Where low begins: MedlinePlus treats a total B12 below 299 pg/mL as a possible sign of deficiency, (MedlinePlus, B12 level) while NICE treats below 180 ng/L as confirmed deficiency and 180 to 350 as indeterminate. (NICE NG239, 2024)
- The first test: MedlinePlus describes total B12 with MMA to confirm, (MedlinePlus, B12 level) while NICE allows total or active B12 first, uses active B12 in pregnancy, and uses MMA or homocysteine first when recreational nitrous oxide use is the suspected cause. (NICE NG239, 2024)
Your doctor decides what your result means for you.
This page does not give supplement, diet, dosing or treatment advice. If you have a flagged result, see after an abnormal screening result for questions to ask.
Questions people ask
What is a normal vitamin B12 level?
MedlinePlus lists 299 to 1,054 pg/mL (219 to 773 pmol/L), and ranges vary between labs. NICE treats a total B12 above 350 ng/L (the same number as pg/mL) as making deficiency unlikely. Use the range printed on your own report. (MedlinePlus, B12 level) (NICE NG239, 2024)
What does a borderline B12 result mean?
NICE calls a total B12 between 180 and 350 ng/L, or an active B12 between 25 and 70 pmol/L, indeterminate: deficiency is possible. It suggests an MMA test may help if you have symptoms. If you have no symptoms, NICE says to seek medical help if symptoms develop. (NICE NG239, 2024)
Can my B12 be normal and still be low in my body?
It can. NICE says to use caution with results in people already taking a product containing B12, because it can raise the blood level without fully correcting a deficiency. If the result suggests deficiency is unlikely but symptoms continue, NICE says to look for other causes and consider repeating the test after 3 to 6 months. (NICE NG239, 2024)
Is a high vitamin B12 level a problem?
MedlinePlus says a high level is uncommon because extra B12 is usually removed in the urine. Conditions that can raise it include liver disease and some blood disorders. Your doctor decides what it means for you. (MedlinePlus, B12 level)
Do I need to fast for a B12 test?
MedlinePlus says not to eat or drink for about 6 to 8 hours before the test. Some medicines can affect the result, but do not stop any medicine without talking to your provider. (MedlinePlus, B12 level)