Vitamin B12 is essential for your health. The vitamin helps to preserve neurological function, maintain red blood cell production, and aid normal DNA synthesis. Vitamin B12 deficiencies can arise when you do not get enough B12 from your diet or when your body can’t use B12 properly. (Langan & Zawistoski, 2011)
Typically, Vitamin B12 deficiencies can be detected through the vitamin B12 test. In patients with normal B12 levels that have clinical symptoms of vitamin B12 deficiency, additional testing may be needed. Specifically, the methylmalonic acid (MMA) test can be used.
Methylmalonic acid is a compound that reacts with vitamin B12 to produce coenzyme A (CoA). Coenzyme A is essential to normal cellular function. When vitamin B12 deficiencies occur, MMA levels increase. Measurement of MMA through the MMA test can provide your doctor with information about an existing vitamin B12 deficiency, especially if the B12 deficiency is mild or just beginning.
The MMA test is more sensitive than the vitamin B12 test, and as a result, is better able to identify vitamin B12 deficiencies at the lower end of the normal range. According to the American Association for Clinical Chemistry, (Lab Tests Online) although the MMA is a very sensitive test for indicating B12 deficiency, there is too much variability for it to provide a clear diagnosis of B12 deficiency. That is why it is often used along with the vitamin B12 test, or to clarify ambiguous vitamin B12 test results. It is often also performed along with the homocysteine test. Homocysteine is an important molecule found in very small amounts in the body. Homocysteine must be metabolized by vitamin B12, so low levels of vitamin B12 (as well as B6 and folic acid) lead to low levels of homocysteine.
The MMA test is not typically ordered as part of a routine physical exam. Your doctor may order the test if the result of your vitamin B12 test is abnormal. In addition, the test may be ordered if you have symptoms of vitamin B12 deficiency. Symptoms of vitamin B12 deficiency include:
- cognitive impairment
- gait abnormalities (abnormal walking, part of a standard neurological examination)
- jaundice (yellowing of the skin or eyes, often seen in people with liver diseases)
- peripheral neuropathy (nerve damage or malfunctioning nerves)
The MMA test may also be ordered if the results of other blood tests are abnormal. For instance, abnormal results from a homocysteine test may prompt your doctor to order the MMA test. The MMA test is also frequently ordered for infants when the doctor suspects the presence of methylmalonic acidemia. Methylmalonic acidemia is a rare genetic disorder in which MMA builds up in the bloodstream.
No specific preparation is needed for the MMA test.
The MMA test is performed on blood taken from a standard blood draw. Typically, a doctor or nurse will take a blood sample from your arm in a clinical setting. The blood will be collected in a tube and will be sent to a lab for analysis. Once the lab reports the results, your doctor will be able to provide you with more information about the results and what they mean.
Patients undergoing the MMA test may experience some discomfort when the blood sample is drawn. Needle sticks may result in pain at the injection site during the test. Following the test, patients may experience pain or throbbing at the injection site. Bruising may also occur after the test is completed.
The risks of the MMA test are minimal. These risks are common to most blood tests. Potential risks for the test include:
- difficulty obtaining a sample, resulting in multiple needle sticks
- excessive bleeding at the needle site
- fainting as a result of blood loss
- the accumulation of blood under the skin, known as a hematoma
- development of infection where the skin is broken by the needle
The results of the MMA test may vary depending on the laboratory that performs the test. In general, normal levels of methylmalonic acid are between 0.08 and 0.56 mmol/L (millimoles per liter). Although higher levels of methylmalonic acid may be an indication of vitamin B12 deficiency, elevated levels may not warrant immediate treatment. Your doctor may want to monitor your MMA levels to determine if your vitamin B12 deficiency is progressing. Your doctor may also order additional testing to determine the cause of the vitamin B12 deficiency. Often, your doctor will order homocysteine and folate tests to help identify the cause of the B12 deficiency.
High levels of MMA in the blood may also be an indication of kidney disease. Kidney disease is often identified through the use of other blood and diagnostic tests. Damage to the kidney can prevent the filtering of MMA from the blood. This can result in increased levels of MMA in the bloodstream. Therefore, the presence of high MMA levels in patients with kidney disease may not indicate the presence of a vitamin B12 deficiency. The results of the test must be discussed with your doctor and evaluated along with other diagnostic tests to determine if a vitamin B12 deficiency is present.