B12 and folate blood tests measure two vitamins that help your body make healthy blood cells. B12 also supports normal nerve function. Low levels may point to a deficiency, while results within the lab’s range are generally reassuring. Doctors interpret both results alongside symptoms and other blood tests, because the two deficiencies can look similar and a single number does not always explain how you feel.
What do B12 and folate blood tests measure?
Vitamin B12 and folate are important for making healthy red blood cells and supporting normal cell function. Vitamin B12, also called cobalamin, helps maintain the nervous system. Folate, also known as vitamin B9, is essential for making DNA and producing new cells.
These vitamins are often measured together because a shortage of either can affect red blood cell production and lead to a type of anaemia. Anaemia occurs when the body has too few healthy red blood cells or insufficient haemoglobin, the protein that carries oxygen around the body.
According to the NHS guidance on diagnosis, healthcare professionals may assess haemoglobin, red blood cell size, and B12 and folate levels together. The results help identify possible deficiencies and determine whether further investigation is needed.
What does a low vitamin B12 result mean?
A low vitamin B12 result may indicate that the body does not have enough of this vitamin to support normal blood cell production and nerve function. The significance depends on the reported level, the laboratory's reference range, any symptoms, and other test findings.
As one reference example, MedlinePlus lists a typical blood B12 range of approximately 299 to 1,054 picograms per millilitre (pg/mL). A result below 299 pg/mL may suggest a deficiency according to that reference. However, laboratories can use different reference ranges, so the range printed on your own report is important.
A lower B12 level can have several possible explanations, including:
- Insufficient dietary intake: People who follow a strict vegan diet without reliable B12 sources may be at risk of inadequate intake.
- Reduced absorption: Conditions affecting the stomach or intestines may make it harder for the body to absorb B12 from food.
- Pernicious anaemia: This is a condition in which the immune system interferes with the body's ability to absorb B12 by affecting a substance needed for its absorption.
Vitamin B12 deficiency may be associated with tiredness, weakness, tingling or numbness in the hands and feet, or problems with balance. Some people experience memory or concentration difficulties. These symptoms can have other causes, and B12 deficiency can sometimes affect the nervous system even when anaemia is not present.
What does a low folate result mean?
A low folate result may indicate that the body does not have enough vitamin B9 to support normal cell growth and red blood cell production. The result can be related to inadequate dietary intake, poor absorption, or other nutritional problems.
MedlinePlus information on folate testing gives a typical blood folate range of approximately 2.7 to 17.0 nanograms per millilitre (ng/mL). A value below the laboratory's reference range may suggest low folate, but the interpretation depends on the test method and clinical context. Recent changes in diet or supplements and certain medicines may also affect test measurements.
Folate deficiency can lead to anaemia in which red blood cells are larger than usual and may not function normally. Possible experiences include fatigue, weakness, a sore or red tongue, and mouth ulcers. However, these symptoms do not establish that folate is low; the blood results and clinical assessment help clarify the cause.
Folate is particularly important before and during pregnancy because adequate intake helps reduce the risk of neural tube defects in a developing baby. Anyone who is pregnant or planning a pregnancy should discuss appropriate folic acid supplementation and any relevant test results with a healthcare professional.
What if both B12 and folate results are low?
When both results are below their reference ranges, this may indicate that the body is not receiving or absorbing enough of both vitamins. Dietary intake, absorption problems, and other clinical factors may need to be considered. The combination does not, by itself, establish why the levels are low.
Doctors may also review a complete blood count to see whether anaemia is present and whether red blood cells are larger than usual. Depending on the findings, they may investigate dietary patterns, digestive conditions, medical history, and medicines that could be relevant.
It is important to assess both vitamins rather than assume that one low result explains every finding. B12 and folate deficiencies can produce similar changes in the blood, but a B12 deficiency can also affect nerves. This distinction matters when deciding what additional tests or treatment may be appropriate.
What does it mean if B12 or folate is borderline?
A borderline result is a value close to the laboratory's lower limit or one that does not clearly establish whether a deficiency is present. It is not necessarily the same as a confirmed deficiency, but it may require further assessment, particularly when symptoms or other blood test results raise concerns.
The Merck Manual's clinical guidance on vitamin B12 deficiency describes additional testing that may help clarify uncertain results. One example is methylmalonic acid (MMA), a substance that may rise when the body has insufficient B12. Homocysteine may also be measured in selected cases, although its level can be affected by more than one vitamin deficiency and other health factors.
Not everyone with a borderline result needs the same follow-up. A clinician will decide whether repeat testing, additional blood tests, or assessment for a possible underlying cause is appropriate.
What does a normal B12 and folate result mean?
Results within the laboratory's reference ranges are generally reassuring because they do not show a clear reduction in the measured vitamin levels. If both results are normal and there are no concerning symptoms, additional testing may not be necessary.
However, a normal result does not always explain persistent symptoms. The timing of the test, recent vitamin supplementation, other blood results, and the reason for testing may affect how the findings are interpreted. If symptoms continue or worsen, a healthcare professional may consider other causes or decide whether further investigation is appropriate.
What does a high B12 or folate result mean?
A B12 result above the laboratory's reference range does not automatically mean that there is a health problem, but an unexpectedly high value may need to be reviewed. MedlinePlus notes that elevated B12 levels can be associated with certain liver diseases and blood disorders. The result should be considered alongside medical history, other test results, and any relevant supplements or medicines.
For folate, the reference range and the circumstances surrounding the test remain important. Recent changes in intake and the use of folic acid supplements may affect measurements. A result outside the laboratory's range should therefore be interpreted in context rather than treated as a diagnosis on its own.
Why is it important to check B12 before treating low folate?
Vitamin B12 and folate deficiencies can cause similar changes in red blood cells, so treating one without recognising the other may leave an important problem unaddressed. Folic acid can improve the blood changes caused by a B12 deficiency without correcting the underlying B12 shortage. As a result, the blood picture may improve while nerve-related problems continue to develop.
The NHS guidance on treatment explains why B12 levels are checked before folic acid treatment is started for folate deficiency anaemia. This is one reason to seek advice about low results rather than starting high-dose supplements without appropriate assessment.
What happens after an abnormal B12 or folate result?
The next step depends on which vitamin is affected, how far the result falls outside the reference range, whether symptoms are present, and what other investigations show. A healthcare professional may recommend further tests to confirm a suspected deficiency or identify its cause.
Depending on the findings, management may include changes to dietary intake, vitamin supplementation, or treatment for an underlying absorption problem. B12 treatment may involve tablets or injections, while folate deficiency may be treated with folic acid. The appropriate approach depends on the cause and individual clinical circumstances.
When reviewing your report, note the measured values, units, and reference ranges. Bring details of any vitamins or supplements you take, along with information about your symptoms and medical history. Seek professional assessment if symptoms persist, worsen, or interfere with daily life. Prompt assessment is especially important for new or progressive numbness, weakness, or balance problems, because these may require investigation.