B12, folate and ferritin are often tested together when someone has tiredness, pins and needles, memory problems or restless legs. Vitamin B12 and folate are vitamins your body needs for healthy blood cells and nerves. Ferritin is a protein that stores iron, and it is the most useful single blood marker of iron stores.
A result inside the lab's range does not always settle the question. NICE guidance for England treats a wide middle band of B12 results as "indeterminate". Ferritin rises with inflammation, so a normal-looking result can hide low iron. This guide explains what each test measures, how results are reported, and when a "normal" result needs a second look.
What the B12 supplement trials show for brain fog and ferritin in restless legs are covered separately.
Only about a fifth of B12 in the blood is in the active form, according to a 2025 review in American Family Physician. NICE says the active B12 test measures the form the body can use. Its committee thought, from experience, that active B12 is more accurate. But it is significantly more expensive.
NICE lists common symptoms and signs of B12 deficiency. They include:
NICE also says B12 deficiency can be linked to symptoms of depression, anxiety or psychosis.
NICE advises offering a test to people with at least 1 common symptom and at least 1 common risk factor. These include a diet low in animal foods without fortified foods or supplements, and a family history of B12 deficiency. Autoimmune conditions such as thyroid disease, type 1 diabetes or coeliac disease also count. So do stomach or bowel surgery, including many weight-loss operations, and recreational nitrous oxide use. Medicines on the list include metformin, proton pump inhibitors, H2 blockers, colchicine, pregabalin and some anti-seizure medicines.
NICE also says deficiency should not be ruled out just because there is no anaemia. The HSE makes the same point: nerve symptoms can appear with a normal full blood count.
The NHS and HSE say B12 or folate deficiency affects around 1 in 10 people aged 75 or over. For ages 65 to 74, it is 1 in 20.
The NHS and HSE list the same broad symptoms for folate deficiency as for B12. These include extreme tiredness, pins and needles, a sore red tongue, mouth ulcers, low mood and problems with memory. In Ireland, HSE laboratory advice says B12 and folate are generally requested together.
The NHS lists tiredness, shortness of breath, palpitations, pale skin and headaches as symptoms of iron deficiency anaemia. Restless legs syndrome is among the less common ones. The NHS says the first test is usually a full blood count. But the American Society of Hematology's 2026 guideline stresses diagnosing iron deficiency whether or not anaemia is present. That is where ferritin comes in.
For the wider list of causes, see our guide to tingling and our guide to tiredness.
NICE sets thresholds for the first B12 test. It also says labs may use their own validated thresholds where these vary substantially.
In the US, the American Family Physician review uses the same total B12 cut-offs, in pg/mL. It describes 180 to 350 as borderline, needing an MMA test.
In Ireland, the HSE's 2025 laboratory advice lists the NICE guideline as relevant material. Each lab sets its own reference range, so check the range on your report.
NICE says there is no "gold standard" B12 test. A 2014 British Society for Haematology guideline says the clinical picture is the most important factor. It also says treatment should not be delayed when strong symptoms clash with the test result. Several things can make a result look better than it is.
NICE says the combined contraceptive pill can lower total B12 without causing deficiency, although a low level may still mean deficiency. The HSE adds pregnancy and hormone replacement therapy. NICE recommends active B12 as the first test in pregnancy.
HSE laboratory advice says raised B12 may be linked with liver disease, chronic kidney failure, and autoimmune or inflammatory disease. It has also been described in some blood cancers and solid tumours. The HSE stresses this is for interpreting a high result. It is not a reason to test B12 to look for these conditions.
NICE says MMA results should be read against the lab's own range, because there are no generally accepted reference ranges. Homocysteine can also rise with folate deficiency. NICE notes the homocysteine sample has to go on ice, so it is usually taken in hospital. Not every lab offers MMA, so availability varies.
The 2014 British Society for Haematology guideline says serum folate is the first-line test. It found it as useful as red cell folate.
Folate and B12 are linked in one important way. The NHS says your GP will check B12 before you start folic acid. Folic acid can improve symptoms enough to mask an underlying B12 deficiency, which could then affect the nervous system.
In Ireland, HSE laboratory advice says Department of Health folic acid advice for anyone who could become pregnant does not depend on a test. No test is needed before following it.
Ferritin is the most useful single test for iron stores, but different bodies use different cut-offs.
The British Society of Gastroenterology says most labs set the lower limit of normal between 15 and 30 µg/L. So a result just above your lab's cut-off may still be low by some of these standards. Ask for the number, not just "normal".
Ferritin is an acute phase protein, which means it rises with inflammation and infection. HSE laboratory guidance says ferritin should be measured when you are well, because it can rise significantly in acute illness. It also suggests measuring C-reactive protein (CRP) alongside to help interpretation. The British Society of Gastroenterology says transferrin saturation can help if a falsely normal ferritin is suspected.
MedlinePlus lists causes of raised ferritin. These include iron overload, inflammation, autoimmune disease, liver disease, cancer, obesity, alcohol use disorder and an overactive thyroid. In Ireland, haemochromatosis matters. HSE laboratory guidance says Ireland has the highest incidence of hereditary haemochromatosis worldwide. It says 1 in 83 people in Ireland are genetically predisposed. It recommends ferritin with fasting transferrin saturation as the first tests. A high ferritin on its own is not a diagnosis.
NICE says B12 symptoms may start to improve within 2 weeks of treatment, but this can take up to 3 months.
These are ordinary blood tests from a vein in your arm. The NHS says a full blood count needs no preparation. MedlinePlus says you may be asked to fast before a ferritin or B vitamin test. HSE guidance says transferrin saturation is best measured fasting. Follow the instructions you are given.
Tell the person taking your blood about every supplement you use, including B12 tablets, sprays, patches or injections and iron. Do not stop a prescribed medicine on your own. Our guide to blood tests worth asking about covers preparing for the appointment.
If low mood is part of the picture and you are struggling, call Samaritans on 116 123 (Ireland and UK). In Ireland you can also text 50808. In an emergency, call 112 or 999.
NICE treats total B12 above 350 ng/L as making deficiency unlikely, and below 180 ng/L as confirming it. Results in between are indeterminate. Your lab may use its own range, so ask your GP what your number means.
Ask your GP whether the result was indeterminate and whether an MMA test makes sense. Mention supplements, the contraceptive pill and any nitrous oxide use.
It depends on the guideline. The British Society of Gastroenterology says below 30 µg/L generally means low stores. The American Society of Hematology now uses 30 ng/mL or lower for adults. With inflammation, higher levels can still hide deficiency.
Ask your GP first. NICE says over-the-counter B12 can raise results without fully treating a deficiency. It advises taking samples before starting prescribed replacement. Always say what you take.
Sometimes. A full blood count needs no fasting, but you may be asked to fast for ferritin, iron studies or B vitamin tests. Follow the instructions you are given.
This article is general information, not medical advice. It cannot tell you whether you need a test or what your result means for you. Talk to your GP, pharmacist or prescriber about your own situation, and before starting or stopping any supplement or medicine.
Reviewed by: Proco Team
Last updated: November 2026