Low B12 Result
This page is about reading the number. For symptoms, causes and treatment, see our vitamin B12 deficiency fact sheet.
Quick answer: Total B12 under 133 pmol/L (180 pg/mL) confirms deficiency, and over 258 pmol/L (350 pg/mL) makes it unlikely. In between is borderline: a methylmalonic acid test or a treatment trial helps. Active B12 under 25 pmol/L is low. Numbness, unsteadiness or memory change means start treatment without waiting.
What the test measures
- Total B12 (serum cobalamin) measures all B12 in blood, including the part bound to a carrier your cells can’t use. It’s the usual first test.
- Active B12 (holotranscobalamin) measures only the usable fraction. It’s the preferred first test in pregnancy, when total B12 falls naturally.
- Methylmalonic acid (MMA) builds up when cells are short of B12. It helps settle borderline results.
Cut-offs (NICE 2024)
- Total B12: under 133 pmol/L (180 pg/mL) deficient; 133–258 pmol/L (180–350 pg/mL) borderline; over 258 pmol/L deficiency unlikely.
- Active B12: under 25 pmol/L deficient; 25–70 borderline; over 70 unlikely.
- Australian and UK labs report pmol/L; US labs use pg/mL (the same as ng/L). Multiply pmol/L by 1.355 to get pg/mL.
- Labs set their own ranges, so a “low” flag can appear at different numbers. People of Black ethnicity may have higher normal B12 levels.
What a low result usually means
Common causes: a vegan or vegetarian diet, long-term metformin or acid-reducing medicines (PPIs), pernicious anaemia (autoimmune gastritis), older age, coeliac or Crohn’s disease, stomach or bowel surgery, and heavy alcohol use.
Important to know: recreational nitrous oxide (“nangs”) inactivates B12 and can cause serious nerve damage even when the blood level looks normal.
Symptoms
Tiredness, anaemia, pins and needles, unsteadiness, memory problems and a sore tongue. Full details in the B12 fact sheet.
What happens next
- Borderline result with symptoms: an MMA test, or a trial of B12 treatment.
- Full blood count and folate, and intrinsic factor antibodies to check for pernicious anaemia.
- Treatment is injections or tablets depending on the cause. Don’t take high-dose folic acid alone until B12 is sorted.
- Repeating B12 blood levels during injections isn’t usually needed; how you feel and your blood count matter more.
See your GP within days, and start treatment without waiting for more tests, if you have numbness, tingling, weakness, unsteady walking or memory changes with a low or borderline B12: nerve damage can become permanent. The same applies if you’re pregnant or breastfeeding with a low result. Get urgent help (000 in Australia, or your local emergency number) for sudden weakness, trouble walking or loss of bladder control, especially after nitrous oxide use.
Practise reading results. Play Panelysis, the daily blood-test puzzle: a fictional patient, a real-looking panel, three guesses. More in medical games.
Common questions
What B12 level is too low?
Total B12 under 133 pmol/L (180 pg/mL) or active B12 under 25 pmol/L confirms deficiency. Levels between 133 and 258 pmol/L (180–350 pg/mL) are borderline, and your doctor will weigh symptoms and may order an MMA test.
Is a B12 of 200 low?
It’s borderline whether your lab uses pmol/L or pg/mL: 200 pmol/L and 200 pg/mL both sit in the grey zone. With symptoms or risk factors, your GP may check MMA or start a trial of treatment.
What’s the difference between total B12 and active B12?
Total B12 measures all B12 in your blood, including a part your cells can’t use. Active B12 measures only the usable fraction, so it can be more accurate, particularly in pregnancy.
Can I have B12 deficiency with a normal result?
Yes. B12 tests aren’t perfect, and some people have deficiency symptoms with a result in the normal or borderline range. If symptoms strongly suggest it, your doctor may check MMA or treat anyway.
How quickly will my B12 come up with treatment?
Injections raise blood levels quickly. Tiredness often improves within days to weeks, while nerve symptoms can take months and may not fully recover, which is why treatment shouldn’t be delayed.
Evidence and more information
- NICE NG239 (2024): Vitamin B12 deficiency in over 16s, diagnosis and management (test cut-offs)
- Pathology Tests Explained (RCPA): Vitamin B12 and folate
- Devalia et al, British Journal of Haematology 2014: BSH guideline on cobalamin and folate disorders
maladywise.com/blood-tests/low-b12/