B12 Blood Test: Normal Levels in the UK and a Low Result

A B12 blood test measures the amount of vitamin B12 in a sample of blood, and there is no single UK cut-off for a low result: at 11 of 12 NHS laboratories the lowest B12 counted as normal, or as deficiency unlikely, runs from 180 to 200 ng/L. Folate is usually measured on the same sample⁶, and a result is read alongside symptoms rather than on the number alone¹⁶.

Below are the ranges 12 NHS laboratories print, what can make a result misleading, what can lower B12 and what the NHS does after a low result. A later section covers how blood testing at our Mayfair clinic includes B12 and folate. This is general information, not advice on your own result: a GP or the clinician who requested the test can interpret it.

What a B12 blood test measures and why a GP orders one

The test measures the amount of vitamin B12 in the liquid part of your blood (the serum), and a GP orders it to find the cause of anaemia or nerve symptoms.⁶ The blood is taken from a vein in the arm⁶. The NHS lists symptoms of B12 or folate deficiency anaemia that include extreme tiredness, pins and needles, and problems with memory, understanding and judgement¹.

The NHS blood tests for B12 or folate deficiency anaemia include the size of red blood cells, the level of B12 and the level of folate². The NHS advises seeing a GP if you think you may have a B12 or folate deficiency, and says a diagnosis and treatment as soon as possible are important¹. It adds that if neurological problems develop they can sometimes be irreversible⁵.

What NHS laboratories print as the normal range for B12

Across 12 NHS laboratories in England and Scotland, 11 count a B12 of 180 to 200 ng/L as the lowest normal result, or as deficiency unlikely, and the upper limit, where one is printed, runs from 660 to 2,000 ng/L. We opened each laboratory's own test page in October 2026 (they are listed in the sources). Eight print one range and four (Bristol, Gloucestershire, Bath and Oxford) print interpretive bands. Oxford, the twelfth, prints bands only and calls deficiency highly unlikely above 350 ng/L. The 12 cover seven areas: the South West, South East, East of England, West Midlands, Yorkshire, London and Scotland. We used no page for Wales, Northern Ireland, the North West, the North East or the East Midlands.

A selection from across the spread, in ng/L:

• Oxford: under 180 deficiency highly likely, 180 to 350 cannot be excluded, over 350 highly unlikely¹⁴
• Gloucestershire: 180 to 914 normal, 145 to 179 indeterminate, 144 or below low¹¹
• Bristol (Severn Pathology): under 145 likely deficiency, 145 to 180 possible deficiency, over 180 not likely¹⁰
• Cornwall: 187 to 883 for ages 19 and over¹³
• Sussex: 197 to 771 for adults¹⁵
• North Midlands: 200 to 900 (printed in pg/mL, which is the same number)¹⁷
• South West London: 190 to 660²⁰
• Edinburgh and the Lothians: 180 to 2,000 for over 15s²¹

The same result can be inside the range at one laboratory and below it at another. As an illustration (not a reading of anyone's result), a B12 of 190 ng/L is inside the printed range at four of the eight single-range laboratories (Cornwall, Birmingham, South West London and Lothian) and below it at the other four (Sussex, York, Peterborough and North Midlands), whose lower limits are 197 or 200^{13, 15, 16, 17, 18, 19, 20, 21}. Among the banded laboratories, 190 is "not likely" to be deficiency at Bristol, normal at Gloucestershire, "unlikely" at Bath and cannot be excluded at Oxford^{10, 11, 12, 14}.

Several laboratories say where their figures come from: Sussex cites a Roche kit insert, Birmingham Abbott, North Midlands Siemens, and York the manufacturer^{15, 17, 18, 19}. A survey of UK laboratories in November 2024 found the total B12 thresholds they applied ranged from 65 to 239 ng/L (48 to 176 pmol/L), with only 16.6% using the 180 ng/L (133 pmol/L) threshold in current UK national guidance⁹. Bath states that this national threshold does not apply to its local method¹².

All 12 pages give B12 in ng/L (two print the equivalent pg/mL). None uses pmol/L, so a result reported in pmol/L needs converting before it is compared with these figures: the 2026 survey pairs 180 ng/L with 133 pmol/L⁹.

Why a B12 result does not always settle the question

A standard B12 test measures total B12 in the blood, which does not always match what the body's cells can use, so a result needs reading with symptoms and other factors.⁶ The NHS describes "functional" B12 deficiency, where levels look normal in the blood but the proteins that carry B12 between cells are not working properly³. A laboratory guide notes that low B12 can also appear on a blood test without any clinical problem, and that methylmalonic acid and homocysteine tests can help confirm a true deficiency⁶.

Laboratory and test-information pages name a few things that can change a result without a deficiency:

• Pregnancy: B12 falls physiologically in pregnancy and usually does not represent deficiency at a biochemical level, according to Gloucestershire, and Oxford says it uses holotranscobalamin, the active form, instead of total B12^{11, 14}.
• Oestrogen-containing contraceptive pills, which can lower B12 without a true deficiency⁶.
• High-dose biotin (above 5 mg a day), which can interfere with the assay: Sussex and York say samples should not be taken until at least 8 hours after the last dose^{15, 19}.

Folate: the other half of the test

Folate is usually measured on the same blood sample, and the nine laboratories whose folate pages we read in full print a range with lower limits of 1.9 to 5.0 µg/L. Sussex starts at 1.9, Bristol, Peterborough, York and North Midlands at 3, Oxford and Birmingham at 3.1, Gloucestershire at 4.0 and South West London at 5.0, while the upper limits printed run from 10 to 26.8 µg/L^{10, 11, 14, 15, 16, 17, 18, 19, 20}. Peterborough explains why the two are reported together: a deficiency of either vitamin can cause megaloblastic anaemia¹⁶. The NHS adds that before folate treatment is started, the GP checks B12 levels are normal⁴.

What can cause low B12, and where low iron fits

The most common cause of B12 deficiency in the UK is pernicious anaemia, a condition in which the immune system attacks healthy cells in the stomach and stops B12 being absorbed from food.¹ The NHS lists other causes: a vegan diet without B12 supplements or fortified foods, conditions or surgery affecting the stomach or intestines, and some medicines (it names acid-reducing medicines and a common type 2 diabetes medicine, and says the GP will be aware of medicines that affect B12 and will monitor where necessary)³. Folate deficiency has different causes, such as a poor diet, coeliac disease, some medicines, pregnancy and losing folate through frequent urination³. The NHS says both deficiencies are more common in older people, affecting around 1 in 10 people aged 75 or over¹.

Low B12 and low iron can occur together, and when they do the size of red blood cells can look normal. One hospital anaemia pathway lists mixed iron and B12/folate deficiency among the causes of anaemia with a normal red cell size, and lists ferritin and B12/folate among the baseline tests for that situation⁷. The NHS likewise says the blood test for iron deficiency anaemia also checks for other types, including B12 or folate deficiency anaemia⁸. Our guide to ferritin levels in the UK covers the iron side.

What happens after a low result

On the NHS, a low result leads to a GP working out the cause, because treatment depends on it.² The GP may arrange further tests, including blood tests for pernicious anaemia, which the NHS says are not always conclusive². The NHS says B12 deficiency anaemia is usually treated with injections, with tablets for deficiency caused by diet, and that folate deficiency is usually treated with daily folate tablets for about 4 months⁴. It describes blood tests around 7 to 10 days and again around 8 weeks after starting treatment⁴. Those NHS pages were last reviewed in February 2023, so a GP will advise on current practice.

Whatever the result, treatment is a decision for the GP or clinician who reviews it.

Getting B12 and folate checked at our Mayfair clinic

Our Well Woman and Well Man profiles both include B12 and folate, and our Vitamin Profile covers vitamin B9 (folate) and B12 with B1, B2, B6, vitamin C and vitamin D, with results within 24 to 48 hours. The blood testing page describes beginning with a complimentary nurse consultation, or opting for a doctor consultation to determine your testing needs, and notes that appointments at home are available. It also says results are reviewed by GMC-registered doctors, with an additional charge for that review.

Treatment is a separate step. The NHS describes it as injections or tablets, and it is decided by the GP or clinician who reviews the result.

Common questions

Can a B12 result be normal and still be a deficiency?

Yes: a total B12 result inside the range does not rule out deficiency, because it does not always reflect the B12 available to the body's cells.⁶ The NHS describes functional B12 deficiency with normal blood levels³, and Peterborough says the most important factor in assessing a B12 result is the clinical picture¹⁶. A GP or the clinician who requested the test can advise on your own result.

Do I need to fast for a B12 blood test?

It depends on the laboratory: Lab Tests Online UK says fasting is not required for a standard B12 test, but two NHS laboratory pages ask for samples from fasting patients.⁶ Sussex asks for fasting samples¹⁵ and Peterborough asks for samples collected after fasting¹⁶. Whoever arranges the test will tell you how to prepare.

How long do B12 results take?

NHS laboratory pages quote turnaround from 24 hours or less to 4 days for B12, which is the time in the laboratory rather than the time until you are contacted. Sussex, Peterborough, North Midlands, Birmingham and York quote 24 hours or less^{15, 16, 17, 18, 19}, Bath 2 days¹², Bristol and Gloucestershire 3 days^{10, 11} and Lothian 4 days²¹. Where a GP requested the test, the surgery will say how the result will reach you.

Sources

1. Vitamin B12 or folate deficiency anaemia, NHS, last reviewed 20 February 2023
2. Vitamin B12 or folate deficiency anaemia: diagnosis, NHS, last reviewed 20 February 2023
3. Vitamin B12 or folate deficiency anaemia: causes, NHS, last reviewed 20 February 2023
4. Vitamin B12 or folate deficiency anaemia: treatment, NHS, last reviewed 20 February 2023
5. Vitamin B12 or folate deficiency anaemia: complications, NHS, last reviewed 20 February 2023
6. Vitamin B12 and folate, Lab Tests Online UK, 2026
7. Anaemia pathway, South Tees Hospitals NHS Foundation Trust, 2026
8. Iron deficiency anaemia, NHS, 2024
9. Variation in total B12 test thresholds across the UK, Clin Chem Lab Med, 2026 (UK NEQAS participants, November 2024)
10. Severn Pathology, North Bristol NHS Trust, viewed October 2026: vitamin B12, folate
11. Vitamin B12 and serum folate, Gloucestershire Hospitals NHS Foundation Trust, 2026
12. Vitamin B12, Royal United Hospitals Bath NHS Foundation Trust Pathology Services, 2024
13. Vitamin B12, Royal Cornwall Hospitals NHS Trust Pathology Service, 2024
14. Oxford University Hospitals NHS Foundation Trust, 2026: vitamin B12, folate
15. University Hospitals Sussex NHS Foundation Trust Pathology, 2025: B12, folate
16. Peterborough Pathology Services, viewed October 2026: vitamin B12, folate
17. University Hospitals of North Midlands NHS Trust, viewed October 2026: vitamin B12, folate
18. University Hospitals Birmingham NHS Foundation Trust, 2026: vitamin B12, folate
19. York and Scarborough Teaching Hospitals NHS Foundation Trust, viewed October 2026: vitamin B12, folate
20. South West London Pathology, viewed October 2026: vitamin B12, folate
21. Vitamin B12, Edinburgh and Lothians Laboratory Medicine, NHS Lothian, viewed October 2026