Understand your tests / Guide
Vitamin B12 and folate: what the tests can tell you
The roles of B12 and folate, reasons for testing, and how borderline results, diet and supplements affect interpretation.
Key takeaway
B12 and folate support cell formation. Their results are considered with symptoms, the blood count, diet and supplement use; a borderline value may need further investigation.
Two vitamins connected with blood and cells
Vitamin B12, also called cobalamin, is needed for red blood cell formation, DNA production and nervous system function. A blood measurement helps assess vitamin B12 status. It is not a direct measurement of energy and cannot, on its own, establish why someone feels tired or which treatment would help them.
Folate is vitamin B9 and is involved in cell division and the production of genetic material. Folic acid is a form used in fortified foods and supplements. A deficiency of either B12 or folate can be associated with anaemia, so the tests are often requested together, although they measure two different vitamins.
When testing can be helpful
A doctor may request these tests when symptoms or changes in the blood count suggest a possible vitamin deficiency. Tiredness, weakness or numbness in the hands and feet can be part of the assessment. These symptoms also have other causes; they do not establish a deficiency or turn a list of tests into a substitute for consultation.
Personal history matters too: a restricted diet, conditions that impair absorption, digestive surgery and certain medicines can affect vitamin status. The doctor selects tests according to the clinical question. There is no single reason for everyone to have an identical panel or repeat these measurements at the same interval, regardless of their circumstances.
Eating a vitamin and absorbing it are separate steps
B12 occurs naturally mainly in animal foods, including fish, meat, eggs and dairy products; some plant foods are fortified. Absorption involves a process in the stomach and intestine. Someone may therefore struggle to absorb B12 despite eating foods that contain it. The blood measurement alone cannot distinguish this from an insufficient intake.
Folate is found in leafy green vegetables, pulses and other foods. Serum folate is influenced by recent dietary intake and does not necessarily reflect longer-term status. Folate measured in red blood cells can represent a longer period. These are different measurements, so their numbers should not be compared as though they came from the same test.
Preparation starts with your medicines and supplements
For the blood test, a sample is taken from a vein. MedlinePlus notes that fasting may be required for some B vitamin measurements; confirm the instructions for the tests you will have together. Follow the preparation guidance supplied by the laboratory carrying out your tests, rather than assuming that another test's instructions apply.
Tell the team about B12, folic acid, multivitamins or injections you use and when you took them. NICE recommends collecting diagnostic B12 samples before replacement begins, when clinically appropriate. Supplements can raise the measured value without fully correcting a deficiency. Do not stop prescribed treatment or delay medical care simply to obtain a sample.
A borderline result needs context
Check whether your report measures total or active B12, along with its units and laboratory range. NICE recognises an indeterminate group of results: these neither clearly confirm nor exclude deficiency. The absence of anaemia or enlarged red blood cells also cannot, by itself, rule out B12 deficiency. The result and symptoms need to be considered together.
When symptoms and a borderline result justify further investigation, a doctor may request methylmalonic acid, or MMA. This can help clarify the picture, but it also rises with impaired kidney function. Homocysteine is another, less specific marker because folate and other factors influence it too. These measurements also need to be interpreted in their medical context.
From the number to the cause and follow-up
If a value is low, the next question is why: insufficient intake, impaired absorption or another factor. A result within the reference range does not explain all symptoms, and further assessment may be needed. Bring the complete report, any available blood count and a list of products you use so that the results can be discussed together.
Large amounts of folic acid can improve the anaemia associated with B12 deficiency without correcting nerve damage. Supplement choice should therefore not depend on a folate number alone. Treatment and follow-up are guided by the cause and symptoms, with a clear plan for what is expected to improve and what will need reassessment.
Sources
- Vitamin B12 — Fact Sheet for Health Professionals ↗NIH · Office of Dietary Supplements · Accessed 15 September 2026
- Folate — Fact Sheet for Health Professionals ↗NIH · Office of Dietary Supplements · Accessed 15 September 2026
- Vitamin B Test ↗MedlinePlus · National Library of Medicine · Accessed 15 September 2026
- Vitamin B12 deficiency in over 16s: diagnosis and management · NG239, recommendations ↗NICE · Accessed 15 September 2026
- Vitamin B12 level ↗MedlinePlus · National Library of Medicine · Accessed 15 September 2026
- Methylmalonic Acid (MMA) Test ↗MedlinePlus · National Library of Medicine · Accessed 15 September 2026
- Homocysteine Test ↗MedlinePlus · National Library of Medicine · Accessed 15 September 2026
- Vitamin B12 or folate deficiency anaemia · Causes ↗NHS · Accessed 15 September 2026
- Vitamin B12 or folate deficiency anaemia · Diagnosis ↗NHS · Accessed 15 September 2026
- Vitamin B12 or folate deficiency anaemia · Treatment ↗NHS · Accessed 15 September 2026
An editorial explanation referring to the listed sources. General information; individual interpretation and treatment require clinical assessment.
Updated 15 September 2026. How we prepare our content