Methylmalonic acid, for borderline B12 deficiency
Methylmalonic acid test to clarify possible vitamin B12 deficiency when the initial figure has fallen in an indeterminate range. It is a functional marker that rises when cells do not have enough B12, although it can also rise if kidney function is reduced.
No prescription or appointment needed
Results in 7-10 working days
Includes: Methylmalonic acid. See what it measures
What it is for
It is used as a second step when total vitamin B12 neither confirms nor excludes a deficiency. NICE considers the range between 180 and 350 ng/l as a guide to be indeterminate, and recommends considering methylmalonic acid when the clinical context leaves the question open.
It is more specific for B12 deficiency than homocysteine, but it is not interpreted on its own either. Age and, above all, reduced kidney function can raise it without any lack of B12.
When it is recommended
- A recent vitamin B12 result in an indeterminate range, as a guide between 180 and 350 ng/l
- Symptoms compatible with B12 deficiency and an inconclusive total figure
- Disagreement between vitamin B12, homocysteine and the full blood count
- It does not usually make sense as a first test if vitamin B12 has not yet been measured
Clinical comment
We have kept methylmalonic acid as a separate test because the guidelines place it after an indeterminate vitamin B12 result, not as part of everyone's initial assessment. Always including it would make the blood test more expensive without adding an answer in most cases.
Homocysteine is more accessible and can give guidance, but it also rises with folate deficiency and other situations. Methylmalonic acid adds more specificity when the first assessment has not been enough.
Bear in mind that a raised result with kidney failure can be difficult to interpret. Nor is it advisable to delay treatment just to wait for this test when there are significant neurological symptoms and high clinical suspicion; that decision is for the doctor.
Dr. Antonio Mas
What is tested
• Methylmalonic acid: A substance whose metabolism depends on vitamin B12. When B12 is lacking, it can build up and provide evidence of functional deficiency.
- Methylmalonic acid
How to prepare
No fasting is required. If you are already taking vitamin B12, say so before the test, because supplementation can change the result. Do not stop a prescribed treatment without discussing it first.
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Without another needle. It is offered when you order, so you do not have to decide now.
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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)
Recommends measuring serum methylmalonic acid when the vitamin B12 result falls in the indeterminate range, between 180 and 350 ng/l, and the clinical situation does not clarify the picture (recommendation 1.3.12).
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.