Vitamin B12 is usually considered low when serum B12 is below about 200 ng/L, which is the same as 200 pg/mL or about 148 pmol/L. Values between roughly 200 and 300 ng/L are a grey zone where deficiency is possible but not certain, and further tests or symptoms decide. Common causes are a diet without animal foods, pernicious anaemia and long-term use of some medicines.
| Category | Reference range |
|---|---|
| Deficiency likely | below 200 (below 148) ng/L = pg/mL (pmol/L in brackets) |
| Indeterminate | 200–300 (148–221) ng/L = pg/mL (pmol/L in brackets) |
| Typical reference range | about 200–900 (148–664) ng/L = pg/mL (pmol/L in brackets) |
| Active B12 (holotranscobalamin), low | below about 25–35 pmol/L ng/L = pg/mL (pmol/L in brackets) |
Ranges and units differ by lab; use the range on your report. To convert pg/mL to pmol/L, multiply by 0.738.
UK labs often report B12 in ng/L, US labs in pg/mL. These are the same unit, so a value of 250 means the same in both. Some labs, in the UK and elsewhere, use pmol/L instead, where the numbers look smaller: 250 pg/mL is about 185 pmol/L. Check the unit before comparing with any threshold.
Serum B12 is an imperfect test. It can look normal when cells are actually short of B12, and low when they are not, for example in pregnancy or with oral contraceptives. That is why doctors may add active B12 (holotranscobalamin) or methylmalonic acid (MMA) when the result is borderline.
A B12 of about 200 pg/mL (ng/L) sits on the boundary. With symptoms such as tingling, numbness or a large-cell (high MCV) anaemia, a doctor is likely to treat it as deficiency or test further. Without symptoms and with a normal blood count, the usual step is to look for causes and repeat the test, or to check active B12 or MMA.
Values well under 150 pg/mL are much more likely to reflect real deficiency and are usually acted on.
Symptoms develop slowly and can be vague. Some come from anaemia, others from effects on the nerves, and nerve symptoms can appear even when the blood count is normal.
A low or borderline B12 should be discussed with your GP or doctor, who will look for the cause before treatment, because some causes need lifelong management. Ask for an urgent appointment, or use NHS 111, urgent care or the emergency room, if you have:
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No need to decode the numbers yourself: take a photo of the report and the app explains “Low vitamin B12” and the rest in plain words — what is normal and what deserves attention.
App StoreGoogle PlayWeb versionMost labs use a range of roughly 200 to 900 pg/mL (ng/L), or about 148 to 664 pmol/L. Values under 200 suggest deficiency and 200 to 300 is a grey zone.
It is borderline. Whether it counts as deficiency depends on symptoms, the blood count and sometimes extra tests such as active B12 or methylmalonic acid.
Often tiredness and lack of energy, then pins and needles in the hands or feet, a sore tongue, or problems with memory or mood. Some people have no symptoms early on.
Low mood, irritability and poor concentration can be part of B12 deficiency. These symptoms have many causes, so they are assessed together with the blood result.
No, they are unrelated, but both are common and are often found together in people with limited diets or absorption problems. Each is assessed on its own.
This material is for general information only. It is not a medical opinion and does not replace a consultation with a doctor. Reference ranges depend on the laboratory and the method — rely on the range printed on your own report. Your doctor makes the final interpretation.
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