Vitamin B12 Levels: Normal Range, Deficiency Chart and High B12
Most B12 blood tests measure total B12 (serum cobalamin). UK guidance (NICE, 2024) reads below 180 ng/L (133 pmol/L) as confirmed deficiency, 180–350 ng/L as indeterminate, and above 350 ng/L (258 pmol/L) as deficiency unlikely. A 2025 US review uses the same cut-offs in pg/mL, with a methylmalonic acid (MMA) test in between. Above the range, the first question is whether you take supplements or have injections.
Units: pg/mL (US) and ng/L (UK) give the same number. Multiply by 0.738 for pmol/L, so 350 pg/mL is about 258 pmol/L. Active B12 is reported in pmol/L.
Where does my B12 sit?
Type the number from your report and pick its unit. Everything is worked out on your device, and nothing is saved or sent anywhere.
Vitamin B12 levels chart
Log scale. Top numbers are pg/mL (the same as ng/L), bottom numbers pmol/L. The rows show where guidelines and labs draw their lines.
Sources: NICE NG239 (2024); Patel and McGuirk, AFP 2025; NIH ODS; Mayo Clinic Laboratories; BSH 2014; Labcorp test 001503; Gloucestershire Hospitals NHS. Full references below.
| Total B12, pg/mL (= ng/L) | Total B12, pmol/L | Active B12, pmol/L | NICE 2024 reading | What it often points to |
|---|---|---|---|---|
| Below 180 | Below 133 | Below 25 | Confirmed deficiency | Low B12. The next question is why. |
| 180 to 350 | 133 to 258 | 25 to 70 | Indeterminate: possible deficiency | A gray zone. Symptoms decide the next step. |
| Above 350 to 914* | Above 258 to about 675* | Above 70 | Deficiency unlikely | B12 is usually not the cause of symptoms |
| Above 914* | Above about 675* | No upper cut-off | Above the usual range | Supplements or injections first. If not, a calm look at liver, kidneys and blood count. |
*Upper limits vary: 914 at Mayo Clinic Laboratories and several NHS labs, 1245 at Labcorp. NICE sets no upper cut-off, and allows labs to use their own validated thresholds (recommendation 1.3.9).
Normal B12 levels: lab ranges in the US and UK
| Lab | Range or bands |
|---|---|
| Labcorp (US) | 232–1245 pg/mL |
| Mayo Clinic Laboratories (US) | 180–914 ng/L. Below 150 is deficiency, 150–400 borderline. |
| Oxford University Hospitals (NHS) | Below 180 ng/L deficiency highly likely; 180–350 cannot be excluded; above 350 highly unlikely |
| Gloucestershire Hospitals (NHS) | 180–914 ng/L normal; 145–179 indeterminate; 144 or less low |
| North Bristol (NHS) | Below 145 ng/L likely deficiency; 145–180 possible; above 914 high |
So a B12 of 240 can be fine, borderline or low, depending on the lab. Read your lab’s comment as well as the number.
What a B12 test actually tells you
Think of total B12 as a headcount of every B12 molecule in your blood, working or not. Only about 20% travels in the active form, holotranscobalamin, which your cells can use (AFP 2025). Active B12 counts just that working share.
Your body needs B12 to make red blood cells and keep nerves healthy. When cells run short, methylmalonic acid (MMA) builds up. Mayo Clinic Laboratories calls a raised MMA more specific for B12 deficiency at the cell level, and folate deficiency doesn’t raise it. Homocysteine rises too, but it also rises with low folate (NICE 1.3.11).
Active B12 and MMA: the follow-up tests
- First test: NICE (2024) accepts total or active B12, but uses active B12 in pregnancy, and homocysteine or MMA if nitrous oxide is the suspected cause (1.3.1 to 1.3.3).
- Active B12 cut-offs: below 25 pmol/L confirmed deficiency, 25–70 indeterminate, above 70 unlikely (NICE).
- MMA: NICE suggests it for symptoms with an indeterminate result (1.3.12), read against the lab’s own range. Labcorp’s range is 0–378 nmol/L. NIH ODS says above 0.271 µmol/L (271 nmol/L) suggests deficiency, but MMA also rises with kidney problems and age.
Do you need a B12 test?
NICE, and a 2025 US review, advise testing when you have at least one typical symptom and one risk factor (1.2.5). Typical symptoms include unexplained tiredness, memory problems, a sore tongue, pins and needles, balance problems and blurred vision (NICE Box 1).
What your B12 number means
Each band is a case file: clues, usual suspects, the next lead and questions to ask.
Total B12 below 180 pg/mL or ng/L (133 pmol/L), or active B12 below 25 pmol/L
NICE reads this as confirmed deficiency, and a 2025 US review agrees. The longer it goes untreated, the higher the chance of permanent nerve damage (NHS).
- Tiredness, a sore red tongue, mouth ulcers (NHS)
- Pins and needles, numbness, balance problems (NICE; NHS)
- Memory problems or low mood
- Anemia with large red cells, or no anemia at all
- Pernicious anemia (autoimmune gastritis)
- A vegan or low-B12 diet
- Metformin, PPIs or H2 blockers
- Stomach or bowel surgery
- Recreational nitrous oxide
The cause: intrinsic factor antibodies, then a coeliac test if still unknown (NICE 1.4.1, 1.4.6). Plus hemoglobin and folate.
NICE: test before starting B12, and don’t delay treatment for suspected megaloblastic anemia with nerve symptoms (1.3.6). Your doctor will advise the dose.
Ask: What’s causing this? Do I need an intrinsic factor antibody test? Tablets or injections, and for how long?
Total B12 between 180 and 350 pg/mL (133 to 258 pmol/L), or active B12 between 25 and 70 pmol/L
NICE calls this an indeterminate result, meaning possible deficiency. It set the cut-offs from expert experience, accepting some false alarms. In this gray zone, your symptoms matter most.
With symptoms, an MMA test (NICE 1.3.12). The 2025 US review advises MMA for 180–350 pg/mL, and NIH ODS for 150–399 pg/mL.
NICE 1.3.14: consider treatment now for symptoms that could worsen quickly (such as balance problems or anemia), a lasting cause such as autoimmune gastritis, past stomach or bowel surgery, or pregnancy or breastfeeding. No symptoms? Get help if they develop (1.3.15).
Ask: Should we check MMA? Do my symptoms or medicines justify treatment now? When should we recheck?
Total B12 above 350 pg/mL (258 pmol/L), or active B12 above 70 pmol/L
NICE says deficiency is unlikely. If symptoms continue, it advises looking for other causes and considering a repeat test 3 to 6 months later (1.3.16).
- Intrinsic factor antibodies in pernicious anemia can make some assays read falsely normal (BSH 2014; Mayo)
- Nitrous oxide inactivates B12, so levels can look normal (NICE)
- Supplements can raise the level without fixing a deficiency (NICE 1.3.8)
Other suspects: low ferritin and iron studies, thyroid (TSH), vitamin D, folate, sleep and mood. See our “Always tired?” case file.
Ask: What else could explain how I feel? Could anything be hiding a deficiency?
Total B12 above 914 pg/mL (about 675 pmol/L), or above your lab’s range
B12 has no US upper intake limit “because of its low potential for toxicity” (NIH ODS). A high result is still a clue worth explaining.
- B12 supplements or injections
- Liver disease (North Bristol NHS; Mayo)
- Kidney failure (Mayo)
- A myeloproliferative disorder, a group of bone marrow conditions (North Bristol NHS; Mayo)
If you don’t take B12: a full blood count, as a normal one rules out a myeloproliferative disorder (North Bristol NHS). Also liver tests and kidney function.
NICE says nothing on high B12. A 2025 US review notes levels above 1,000 pg/mL on 2 tests have been linked with some cancers and heart deaths. That link doesn’t prove B12 causes them.
Ask: Could my supplements explain this? Is my full blood count normal? Do my liver and kidney tests look OK?
B12 with MMA, homocysteine and blood count: the patterns
B12 is one witness. MMA and homocysteine tell you whether to believe it.
| Total or active B12 | MMA | Homocysteine | Usual pattern |
|---|---|---|---|
| Low | High | High | B12 deficiency confirmed at cell level |
| Indeterminate | High | Any | B12 deficiency likely, if kidney function is normal |
| Indeterminate | Normal | Normal | B12 deficiency less likely |
| Normal | High | High | Possible hidden deficiency (antibody interference or nitrous oxide), or kidney disease raising MMA. Needs a doctor’s review. |
| Low | Normal | Normal | Possible false low: pregnancy, the combined pill, or a paraprotein (an abnormal antibody, as in myeloma) |
| Normal | Normal | High | Points away from B12. Folate deficiency is one cause. |
Large red cells with anemia call for both B12 and folate, ideally measured together (Mayo). Sources: NICE NG239; NIH ODS; Mayo Clinic Laboratories; BSH 2014.
What can throw off a B12 result
Rule out a planted clue first:
- Supplements and injections raise B12 “without fully treating a deficiency” (NICE 1.3.8). Mayo Clinic Laboratories says not to test within 2 weeks of an injection.
- The combined pill can lower total B12 without causing deficiency, though a low result may still be real (NICE 1.3.8).
- Pregnancy. Total B12 falls by about 30% by the third trimester in a normal pregnancy (BSH 2014). See pregnancy.
- Intrinsic factor antibodies. In pernicious anemia, high levels of these antibodies can interfere with some B12 assays and give a falsely normal result (BSH 2014; Mayo).
- Nitrous oxide inactivates B12, so total and active B12 “may therefore appear to be normal even when they have a deficiency” (NICE).
- Paraproteins. An abnormal antibody, as in myeloma, can interfere with the assay and give a low B12 (BSH 2014).
- Biotin. Labcorp asks you to stop high-dose biotin at least 72 hours before the sample.
Usual suspects behind low B12
- Pernicious anemia (pernicious anaemia in the UK, or autoimmune gastritis), where the immune system attacks stomach cells. It’s the most common cause in the UK (NHS), and of clinically evident deficiency worldwide (NIH ODS). A negative antibody test doesn’t rule it out (NICE 1.4.3).
- Diet: vegan diets or others low in animal foods, without fortified foods or supplements (NICE; NIH ODS).
- Medicines: metformin, proton pump inhibitors (PPIs), H2 blockers, colchicine, pregabalin, topiramate, phenobarbital and primidone (NICE Box 2). The MHRA (2022) says low B12 may affect up to 1 in 10 people taking metformin for diabetes.
- Surgery: stomach removal, removal of the last part of the small bowel, and many weight-loss operations (NICE).
- Other conditions: coeliac (celiac) disease, past pelvic or abdominal radiotherapy, or autoimmune disease such as thyroid disease, in you or your family (NICE).
- Recreational nitrous oxide (laughing gas). NICE advises stopping it.
- Older age, partly because atrophic gastritis reduces stomach acid and intrinsic factor (NIH ODS; AFP 2025).
B12 in pregnancy and breastfeeding
Total B12 falls in pregnancy “even when there is no deficiency” (NICE), by about 30% by the third trimester (BSH 2014). So NICE uses active B12 as the first test in pregnancy (1.3.2).
- With an indeterminate result, NICE suggests considering treatment if you’re pregnant or breastfeeding (1.3.14).
- Anemia that isn’t improving on iron during pregnancy is one of NICE’s listed signs (Box 1). See low ferritin.
Your midwife or doctor will advise on testing and treatment.
Older adults, ethnicity and metformin
- Older adults: MMA runs higher with age and kidney problems (NIH ODS), so it’s read alongside kidney function.
- Black ethnicity: NICE notes people of Black ethnicity may have a higher B12 reference range than people of White or Asian ethnicity (1.3.13).
- Metformin: the MHRA advises testing if deficiency is suspected, and considering periodic checks if you have other risk factors.
- Under 16: NICE’s cut-offs don’t apply. Ask your child’s clinician.
Two B12 myths
“No anemia, so it can’t be B12.” NICE says not to rule out deficiency because there’s no anemia or large red cells (1.2.3).
“Folic acid covers it.” Large amounts of folate “can correct the megaloblastic anemia, but not the neurological damage” of B12 deficiency (NIH ODS). When both are low, B12 treatment starts before folate (BSH 2014). More in folate deficiency.
Reading B12 on your lab report or app
- Names: vitamin B12, cobalamin, serum B12 or total B12. Active B12 may appear as holotranscobalamin or holoTC. MMA is methylmalonic acid.
- Units: pg/mL (Labcorp) and ng/L (Mayo, UK labs) give the same number. Some labs use pmol/L. MMA comes in nmol/L or µmol/L (1 µmol/L = 1,000 nmol/L).
- Flags: “H” or “L” only means outside that lab’s range. A B12 of 250 pg/mL carries no flag at Labcorp but is indeterminate under NICE.
- Apps and portals: US portals often show results before your clinician comments. The UK NHS App may add a GP note.
Vitamin B12 pg/mL to pmol/L
Multiply pg/mL (or ng/L) by 0.738 for pmol/L, or divide pmol/L by 0.738 (Labcorp). For ng/mL, multiply by 1,000 to get pg/mL. Or use our blood test unit converter.
| pg/mL = ng/L | pmol/L | Where it sits |
|---|---|---|
| 150 | 111 | Mayo deficiency line; start of the NIH ODS MMA zone |
| 180 | 133 | NICE 2024 and AFP 2025 deficiency line |
| 200 | 148 | Old BSH 2014 cut-off |
| 232 | 171 | Labcorp’s lower limit |
| 350 | 258 | Top of NICE’s indeterminate band |
| 400 | 295 | Top of Mayo’s borderline band |
| 914 | 675 | Upper limit at Mayo and several NHS labs |
| 1245 | 919 | Labcorp’s upper limit |
When to get help urgently
- Chest pain, especially with sweating, feeling sick, light-headed or short of breath: call 911 (US) or 999 (UK).
- Sudden weakness or numbness down one side of the body, a drooping face or slurred speech: call 911 or 999. These are stroke signs.
- Numbness, pins and needles, unsteadiness, trouble walking, bladder problems or vision changes that are new or getting worse, including after nitrous oxide use: contact your doctor the same day (in the UK, NHS 111 out of hours). NICE says treatment shouldn’t wait for results when nerve symptoms come with suspected megaloblastic anemia (1.3.6).
- Pregnant with a low or indeterminate B12, or anemia not improving on iron: contact your midwife or doctor promptly.
Vitamin B12 questions people ask
What is a normal B12 level?
Lab ranges run from 180–914 ng/L (Mayo, several NHS labs) to 232–1245 pg/mL (Labcorp). NICE (2024) reads above 350 ng/L (258 pmol/L) as deficiency unlikely.
What B12 level counts as deficient?
Below 180 pg/mL or ng/L (133 pmol/L), by NICE 2024 and a 2025 US review. US labs often flag below 200 to 250 pg/mL (NIH ODS). Between 180 and 350 is a gray zone.
Is a B12 of 250 low?
It’s in NICE’s indeterminate band (180–350 pg/mL), even if your lab doesn’t flag it. With symptoms, the usual next step is an MMA test.
How do I convert B12 from pg/mL to pmol/L?
Multiply by 0.738. So 300 pg/mL is about 221 pmol/L. To go back, divide by 0.738. pg/mL and ng/L are the same number.
What does high B12 mean?
Supplements or injections are the first thing to check. If you don’t take B12, labs suggest checking for liver disease or a myeloproliferative disorder, which a normal full blood count rules out (North Bristol NHS).
Can you have B12 deficiency with a normal level?
Sometimes. Intrinsic factor antibodies can make some assays read falsely normal, nitrous oxide inactivates B12, and supplements can raise the number without fixing a deficiency.
Does metformin lower B12?
It can, in up to 1 in 10 people, more with higher doses and longer use (MHRA). Testing is advised if deficiency is suspected.
How we wrote this page
Written and medically reviewed by Dr. Mustafa Sakr, MBBCh. Last reviewed: October 2026. The numbers come from current guidelines (NICE NG239 2024, which replaced BSH 2014), a 2025 US review, NIH ODS, the MHRA and published lab ranges, listed below. We update this page when guidance changes.
How the checker works: it compares your total B12 with NICE NG239 cut-offs of 180 and 350 pg/mL or ng/L (133 and 258 pmol/L). Above 914 pg/mL (about 675 pmol/L), or your lab’s upper limit, shows as above the usual range. Your lab’s lower limit adds a note but doesn’t change the NICE band. Active B12 uses 25 and 70 pmol/L, and conversions use 0.738. Symptom and risk boxes change the next steps, following NICE. Numbers that look like the wrong unit or test get a hint instead. Nothing is stored or sent, and the checker can’t weigh your full history.
References
- NICE. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239), 6 March 2024. nice.org.uk/guidance/ng239 · PDF
- Devalia V, Hamilton MS, Molloy AM (BCSH). Guidelines for the diagnosis and treatment of cobalamin and folate disorders. Br J Haematol 2014;166:496–513. Archived by BSH in 2024, superseded by NG239. doi:10.1111/bjh.12959 · b-s-h.org.uk
- Patel H, McGuirk R. Vitamin B12 Deficiency: Common Questions and Answers. Am Fam Physician 2025;112(3):294–300. aafp.org
- NIH Office of Dietary Supplements. Vitamin B12 Fact Sheet for Health Professionals, updated July 2025. ods.od.nih.gov
- NIH Office of Dietary Supplements. Folate Fact Sheet for Health Professionals. ods.od.nih.gov
- MHRA. Metformin and reduced vitamin B12 levels: new advice for monitoring patients at risk. Drug Safety Update, June 2022. gov.uk
- Mayo Clinic Laboratories. Vitamin B12 Assay, Serum (test ID B12). mayocliniclabs.com
- Labcorp. Vitamin B12, test 001503; SI Unit Conversion Table. Test · Conversion table
- Labcorp. Methylmalonic Acid, Serum or Plasma, test 706961. labcorp.com
- NHS labs: Oxford University Hospitals; Gloucestershire Hospitals; North Bristol NHS Trust (vitamin B12; active B12). OUH · Gloucestershire · North Bristol B12 · North Bristol active B12
- NHS. Vitamin B12 or folate deficiency anaemia; Chest pain; Stroke symptoms. B12 anaemia · Chest pain · Stroke
This page is for education only and isn’t medical advice, a diagnosis or a substitute for your own doctor, who knows your history. Reference ranges vary between laboratories. Always interpret results with a qualified clinician. See our medical disclaimer.
