Single marker guide · Vitamin B12 (cobalamin)
Low B12 causes tiredness, brain fog, pins and needles and low mood, and it is one of the few deficiencies that can do lasting nerve damage if it goes on long enough. The catch is that the standard serum test has a wide grey zone, so a "normal" result can still be a problem. Here is what the test measures, what level to aim for, and how to read it against the markers that actually explain it.
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The short answer
The standard B12 test measures total cobalamin in serum, reported in ng/L in the UK (some labs use pmol/L). Below roughly 180 to 200 ng/L is deficient. The trouble is the band between about 180 and 350, which most laboratories label normal but where a meaningful share of people have a functional deficiency: their blood level looks acceptable while their nerves and red cells are short of usable B12. Most clinicians who treat B12 problems aim for well above 400 ng/L, because that is where symptoms consistently resolve.
That is why a B12 number needs company. Folate, a full blood count with red cell size, ferritin and thyroid tell you whether B12 is the cause of your tiredness or a bystander, and in the Signature panel homocysteine shows whether your cells are actually getting enough. Testing B12 alone answers a narrow question. Testing it in context answers the one you have.
Reference ranges
| Serum B12 result | Category | What it means |
|---|---|---|
| Below 180 ng/L | Deficient | Treatment needed. If you have neurological symptoms (numbness, tingling, balance problems) this is urgent, because nerve damage becomes harder to reverse the longer it continues. |
| 180 to 350 ng/L | Grey zone | Reported as normal by most labs, but functional deficiency is common here, especially with symptoms. Folate, MCV and homocysteine decide whether this is a problem. |
| 350 to 400 ng/L | Adequate | Usually fine for people with no symptoms and no absorption risk. Worth improving if you are vegan, over 60, or on acid-reducing medication. |
| 400 to 900 ng/L | Optimal | The range most clinicians target when treating symptoms. Comfortably covers nerve, brain and red cell needs. |
| Above 900 ng/L | High | Usually from supplements or injections and not harmful in itself. An unexplained high level without supplementation is worth mentioning to your GP. |
To convert ng/L to pmol/L, multiply by 0.738. Laboratory cut-offs vary slightly; TrueVitals reports show your result against both the laboratory range and the optimal range, with the reasoning written out.
Symptoms
B12 is needed to make red blood cells. Low levels mean fewer, larger, less efficient cells and less oxygen delivered, which feels like heavy, persistent fatigue. Low iron produces the same feeling, which is why both need testing together. Always tired but tests normal?
B12 is essential for the insulating sheath around nerves and for neurotransmitter production. Deficiency shows up as difficulty concentrating, word-finding trouble and a sense of thinking through treacle. In older adults it is a recognised, reversible cause of memory decline.
Tingling in the hands and feet, numb patches, and unsteadiness are the nerve symptoms. They can appear before any anaemia and they are the reason B12 deficiency should never be left to drift: prolonged deficiency can leave permanent nerve damage.
B12 sits in the pathway that makes serotonin and dopamine. Low levels are consistently associated with depression and anxiety, and they are one of the cheapest things to rule out before assuming a mood problem is purely psychological. Anxiety blood test guide.
A smooth, red, sore tongue and recurring mouth ulcers are classic and often overlooked. Pale or slightly yellow skin can accompany them as red cells break down faster.
When deficiency has progressed to anaemia, the heart works harder to move oxygen around. Breathlessness on stairs and a racing or irregular heartbeat are the signs, and they mean the problem has been building for a long time.
Who runs low
B12 is made by bacteria and reaches us through meat, fish, eggs and dairy. There is no reliable plant source, so anyone vegan or largely vegetarian will run down their stores over a few years unless they supplement or eat fortified foods. Vegan blood test guide.
The bigger group, though, is people who eat B12 but cannot absorb it. Absorption needs stomach acid and a protein called intrinsic factor. Both decline with age, which is why one in twenty over-60s is deficient. Acid-reducing medication (omeprazole, lansoprazole and similar) and metformin both lower absorption. Coeliac disease, Crohn's disease and weight-loss surgery damage or bypass the part of the gut where B12 is taken up. Pernicious anaemia, an autoimmune condition that destroys intrinsic factor, is the most common cause of severe deficiency in the UK and is treated with injections for life. And recreational nitrous oxide inactivates B12 outright, which is why nerve damage in young users has become a recognised problem.
Read it in context
A B12 result on its own tells you whether to take a supplement. Read alongside the right markers, it tells you whether B12 is the cause, how long it has been low, and whether your gut can absorb the fix.
B12 and folate work as a pair in the same chemical pathway, and both cause the same large-red-cell anaemia. Testing one without the other risks treating the wrong deficiency. Taking folate alone can also mask a B12 problem while the nerve damage continues, which is why they are always reported together. Both are in every TrueVitals panel.
Mean cell volume, the average size of your red cells, rises when B12 or folate has been low for months. A high MCV with a grey-zone B12 turns a maybe into a yes. A normal MCV with symptoms means the deficiency is early, which is the best time to catch it.
Iron deficiency is the other common cause of fatigue and can hide a B12 problem by shrinking red cells at the same time B12 is enlarging them, leaving MCV deceptively normal. Checking both is the only way to be sure. Iron blood test guide.
Autoimmune thyroid disease and pernicious anaemia travel together, and an underactive thyroid causes the same fatigue and low mood. If B12 is low and thyroid antibodies are raised, that changes the conversation with your GP. Thyroid blood test guide.
Homocysteine rises when cells lack usable B12 or folate, so it is the functional check that resolves the grey zone: a B12 of 250 with normal homocysteine is probably fine, the same B12 with raised homocysteine is not. It is also an independent cardiovascular risk marker. Included in the Signature panel.
Undiagnosed coeliac disease damages the small intestine and quietly causes B12, folate and iron deficiency together. If two or three of those are low at once, the coeliac antibodies in the Ultimate panel often explain why. Gut health blood test guide.
Which test
Advanced
74 biomarkers · £269
B12 and folate with a full blood count including MCV, full iron studies, full thyroid with Free T3 and antibodies, vitamin D, liver, kidney, lipids and hormones. The right choice if tiredness or brain fog is the question.
View AdvancedMost chosen
Ultimate
114 biomarkers · £349
Everything in Advanced plus coeliac screening, immunoglobulins, complete hormones, fasting insulin and HOMA-IR, Lp(a), tumour markers and urinalysis. The panel that finds why B12 is low, not just that it is.
View UltimateSignature
230 biomarkers · £799
Processed by Randox. Adds homocysteine, the functional B12 check, plus extended cardiovascular, autoimmune, digestive and neurological screening. The deepest private panel in the UK.
View SignatureClinic draw £19 at 103 UK clinics. At-home phlebotomist £39 across 200+ areas. Fast from 10pm the night before. Pay monthly available.
Honest comparison
If you are already on B12 injections or a daily supplement and simply want to confirm your level is holding, a standalone finger-prick B12 test from Medichecks costs around £39 and does that job. Some providers also sell an active B12 (holotranscobalamin) test for about £49, which measures the fraction your cells can use and is a reasonable choice if you sit in the grey zone and want a second opinion on the same marker.
A panel is the better choice when you are testing because of how you feel, when nobody has ever checked the causes rather than the number, or when you want to stop guessing between B12, iron, thyroid and folate as the reason you are exhausted. The Advanced panel works out at under £4 per marker and the report tells you what each result means and what to do about it. See how every UK provider compares.
After the result
See your GP promptly. They will usually check intrinsic factor antibodies for pernicious anaemia and start treatment. Where absorption is the problem, injections are the standard route because tablets cannot get in. Your report flags this clearly and tells you what to ask for.
If you are vegan or your diet is the likely cause, a daily sublingual methylcobalamin supplement bypasses the absorption step and raises levels reliably, which is why TrueVitals reports recommend it over ordinary tablets. If you take acid-reducing medication, mention it to your GP; the dose or the timing may be adjustable.
B12 stores replenish over weeks and symptoms lag behind the number. Retesting at three months shows whether the route you chose is working and whether MCV and homocysteine have normalised. Your second report compares against your first automatically.
This page is general information, not medical advice. If you have numbness, tingling or balance problems, do not wait for a retest: see your GP now.
FAQs
The standard test measures total vitamin B12 (cobalamin) in serum, reported in ng/L or pmol/L. It reflects how much B12 is circulating, not how much your cells are using, which is why results between about 180 and 350 ng/L need interpreting alongside folate, red cell size and, ideally, homocysteine.
Most UK laboratories call below 180 to 200 ng/L deficient and anything above that normal. Clinicians who treat B12 problems generally aim for above 400 ng/L, because symptoms in the 180 to 350 range are common despite a "normal" label. TrueVitals reports show both ranges.
Yes. Functional deficiency, where the blood level looks acceptable but cells are short, is well documented in the grey zone. Raised homocysteine, a high MCV, or symptoms that respond to supplementation all point to it. This is the main reason a single B12 number is unreliable on its own.
If you want to know your underlying status, stop oral supplements for a few days beforehand, or simply tell us you take them so the report interprets the result correctly. If you are on injections, testing shows whether your maintenance schedule is holding and should not be interrupted.
Not for B12 itself. TrueVitals panels also include glucose, insulin and lipids, so we ask you to fast from 10pm the night before your appointment and drink water as normal.
Yes, if your GP suspects deficiency: anaemia on a blood count, neurological symptoms, a vegan diet, or a condition affecting absorption. Routine testing without those is often declined. If you want the number and its context without the wait, a private panel is the direct route. What to ask your GP for.
A standalone finger-prick B12 test is around £39, and an active B12 test around £49. A TrueVitals Advanced panel is £269 and includes B12 and folate with 72 other markers, a full blood count, iron studies, thyroid and vitamin D, with a venous draw and a fully written report.
For dietary deficiency, sublingual methylcobalamin dissolved under the tongue is absorbed through the mouth lining and does not depend on stomach acid or intrinsic factor, which is why TrueVitals reports recommend it over swallowed tablets. Where absorption is the problem, as in pernicious anaemia, injections from your GP are the standard treatment.
74 to 230 biomarkers. Venous draw at 103 UK clinics or at home. A report that tells you what your results mean and what to do next.
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