Vitamin B12 blood test
Blood test to investigate possible vitamin B12 deficiency, with vitamin B12, folic acid, full blood count and homocysteine. It assesses the B12 figure together with its possible effects on the blood and with a functional marker that can add context when the result falls in a borderline range.
No prescription or appointment needed
Results in 3-5 working days
Includes: Vitamin B12 · Folic acid · Full blood count · Homocysteine. See what each one measures
What it is for
It is used to investigate whether symptoms such as tiredness, tingling, loss of sensation or memory problems may be related to vitamin B12 deficiency. It also helps check people with risk factors, such as a vegan diet, digestive surgery or long-term treatment with metformin or proton pump inhibitors.
The profile distinguishes between the amount of B12 in the blood, possible changes in the full blood count and raised homocysteine. This combination gives better guidance than a single figure, although an indeterminate result may require measuring methylmalonic acid.
When it is recommended
- Tingling, loss of sensation, balance problems or unexplained neurological symptoms
- Anaemia or a raised mean corpuscular volume in a previous full blood count
- A vegan or vegetarian diet with insufficient or uncertain supplementation
- Long-term treatment with metformin or proton pump inhibitors
- Stomach or bowel surgery, coeliac disease or another cause of malabsorption
- A previous borderline vitamin B12 result that requires context
Clinical comment
Total vitamin B12 is a good initial test, but it has a range of indeterminate results. Homocysteine can help in that range, although it also rises with folate deficiency, kidney disease, age and other factors. That is why it should not be read as automatic confirmation.
Methylmalonic acid is more specific for functional B12 deficiency and is kept as a second step when the B12 figure and the rest of the assessment do not allow a conclusion. Keeping it out of this profile avoids making the initial assessment more expensive for people who do not require it.
If the deficiency is confirmed, the cause still has to be investigated. It may be due to diet, medication, digestive surgery or impaired absorption. When autoimmune gastritis or pernicious anaemia is suspected, the assessment should be extended with intrinsic factor and parietal cell antibodies.
Dr. Antonio Mas
What is tested
- Vitamin B12
- Measures the circulating concentration of cobalamin and is the initial test for investigating possible deficiency.
- Folic acid (folate)
- Detects folate deficiency, which can cause changes in the full blood count similar to those of B12 deficiency.
- Full blood count
- Looks for anaemia, enlarged red blood cells and other changes in the blood cells.
- Homocysteine
- Can rise when vitamin B12 or folic acid is lacking. It is sensitive but not specific, because it also changes for other reasons.
How to prepare
Fast for 8 to 12 hours. You can drink water. If you take vitamin B12 supplements or injections, say so before the sample is taken, because they can change the result. Do not stop a treatment prescribed to correct a deficiency without discussing it with your doctor.
You can add
Without another needle. It is offered when you order, so you do not have to decide now.
- General blood testA general blood test with full blood count, glucose, cholesterol and kidney and liver function: the basic parameters included in any general check-up.+€59.00
- Coeliac disease antibodiesMeasures the antibodies used to investigate coeliac disease; for a reliable result, it must be done while you are still eating gluten.+€39.00
- Thyroid functionBrings together TSH and free T4, the two basic tests for assessing how the thyroid is working.+€29.00
Related blood tests
We have other profiles in Digestive with more or fewer tests:
- Lactose intolerance test€60.00
- Sorbitol intolerance test€60.00
- Fructose intolerance test€60.00
- SIBO breath test€64.00
- Helicobacter pylori breath test€69.00
- Coeliac disease blood test (antibodies)€69.00
- Vitamin B12 blood test with antibodies€149.00
- Methylmalonic acid, for borderline B12 deficiency€149.00
- Coeliac disease genetic test (HLA DQ2/DQ8)€229.00
- Gut microbiota test€359.00
The most comprehensive is not always the most useful. Each profile page explains when it makes sense and when it does not.
What it is based on
National Institute for Health and Care Excellence (NICE)
Vitamin B12 deficiency in over 16s: diagnosis and management (NG239)
Places the range in which a vitamin B12 result is indeterminate, and allows no conclusion on its own, between 180 and 350 ng/l. This is the reason for measuring the full blood count and a functional marker in the same sample instead of relying on the B12 figure alone.
Devalia V, Hamilton MS, Molloy AM · British Journal of Haematology 166(4):496-513
Guidelines for the diagnosis and treatment of cobalamin and folate disorders
Places homocysteine as a second-line test when the B12 figure does not clarify the picture, and warns that it is less specific than methylmalonic acid and that folate deficiency also raises it, which is the reason for measuring folate at the same time.
Protocol designed by Dr. Antonio Mas, specialist in Endocrinology and Nutrition, Spanish medical licence no. 080846289, and reviewed in September 2026.
This blood test is a standardised protocol and does not replace a medical consultation. If you have symptoms, a known condition or a complex clinical situation, speak to your doctor.