At a Glance
Why Get Tested?
To help detect early and/or mild vitamin B12 deficiency, especially at the tissue level; to help diagnose methylmalonic acidaemia, a rare inherited metabolic disorder
When to Get Tested?
If you have a vitamin B12 concentration in the low end of the normal range and/or have symptoms of neuropathy, such as numbness and tingling in the hands and feet
Test Preparation Needed?
You may be instructed to fast before sample collection for this test.
The Test Sample
What is being tested?
The methylmalonic acid (MMA) test measures the amount of MMA in the blood or urine. There is no agreement on whether, blood or urine is best for this test and there are interpretative guidelines and reference levels for each. Your doctor may request both a blood and a urine test at the same time.
MMA is a compound produced in very small amounts during amino acid metabolism. Increased production of MMA is a sensitive and early indicator of vitamin B12 deficiency. Normally, vitamin B12 acts as a coenzyme, promoting the conversion of methylmalonyl CoA to succinyl CoA. If there is not enough B12 available to act as a coenzyme, then methylmalonyl CoA concentrations begin to rise, and the body converts the methylmalonyl CoA to MMA instead. Decreased availability of B12 leads to increases in blood and urine MMA levels. Increased production of MMA in newborn babies may suggest one of the inherited problems collectively called the methylmalonic acidaemias.
Over time vitamin B12 deficiency can cause changes to bone marrow function or the usual composition of blood or blood components. These changes include anaemia with a reduced numbers of larger than normal red blood cells. It can also cause signs and symptoms of disease, inflammation or damage to the peripheral nerves (neuropathy), such as numbness and tingling in the hands and feet and/or mental or behavioural changes (confusion, irritability, depression). Increased concentrations of MMA are often detectable before the occurrence of haematological changes. An affected person may not have any symptoms at all or may have some degree of neuropathy.
The relationship between MMA and B12 has been known for about 40 years, but the use of MMA testing is not widespread because doctors and laboratory scientists do not fully agree on its clinical value. Some in the medical community think that MMA may be a better measure of bioavailable B12 than the usual vitamin B12 test. This is because a relatively large amount of the B12 found in the blood is bound to proteins and is not freely available to act in chemical processes. Some health professionals are convinced that MMA and homocysteine (which may be elevated when either B12 or folate are deficient) are valuable in detecting early and mild cases of B12 deficiency. Others argue that many of the mild deficiencies detected do not progress to more severe deficiencies and do not necessarily need to be identified or treated.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in your arm. A single random urine sample may be collected (the second morning sample is preferred) or a 24-hour collection of urine may be requested.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
For the MMA blood test, the patient should be fasting. For a random urine sample, the patient should fast overnight, discard the first urine sample of the morning, and then collect the second sample.
Ask a Laboratory Scientist
NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.