Vitamin B-12 is essential for your health. The vitamin helps to preserve neurological function, maintain red blood cell production, and aid in normal DNA synthesis. Deficiencies can arise when you don’t get enough B-12 from your diet or when your body can’t use it properly.
Typically, vitamin B-12 deficiencies can be detected through the vitamin B-12 test. In people with normal levels who have clinical symptoms of vitamin B-12 deficiency, additional testing may be needed. Specifically, the methylmalonic acid test can be used.
Methylmalonic acid is a compound that reacts with vitamin B-12 to produce coenzyme A (CoA). Coenzyme A is essential to normal cellular function. When vitamin B-12 deficiencies occur, methylmalonic acid levels increase. Measurement of methylmalonic acid through the methylmalonic acid test can provide your doctor with information about an existing vitamin deficiency, especially if the B-12 deficiency is mild or just beginning.
The methylmalonic acid test is more sensitive than the vitamin B-12 test. As a result, it’s better able to identify vitamin B-12 deficiencies at the lower end of the normal range. According to the American Association for Clinical Chemistry, although the methylmalonic acid test is a very sensitive test for indicating B-12 deficiency, there is too much variability for it to provide a clear diagnosis. That’s why it’s often used along with the vitamin B-12 test, or to clarify ambiguous vitamin B-12 test results. It’s also often 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 B-12, so low levels of the vitamin (as well as B-6 and folic acid) lead to elevated levels of homocysteine.
The methylmalonic acid test isn’t typically ordered as part of a routine physical exam. Your doctor may order the test if the result of your vitamin B-12 test is abnormal. In addition, the test may be ordered if you have symptoms of vitamin B-12 deficiency. Symptoms of B-12 deficiency include:
- cognitive impairment
- gait abnormalities (abnormal walking, usually measured during a standard neurologic examination)
- jaundice (yellowing of the skin or eyes, often seen in people with liver disease)
- peripheral neuropathy (nerve damage or malfunctioning nerves)
The methylmalonic acid 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 methylmalonic acid test. The methylmalonic acid test is also frequently ordered for infants when the doctor suspects the presence of methylmalonicacidemia. Methylmalonicacidemia is a rare genetic disorder in which methylmalonic acid builds up in the bloodstream.
No specific preparation is needed for the methylmalonic acid test.
The methylmalonic acid 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.
People undergoing the methylmalonic acid 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, you may experience pain or throbbing at the injection site. Bruising may also occur after the test is completed.
The risks of the methylmalonic acid test are minimal and can occur with any blood test. Potential, but rare, risks include:
- difficulty obtaining a sample, resulting in multiple needle sticks
- excessive bleeding at the needle site
- fainting as a result of blood loss
- accumulation of blood under the skin, known as hematoma
- development of infection where the skin is broken by the needle
The results of the methylmalonic acid 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 B-12 deficiency, elevated levels may not warrant immediate treatment. Your doctor may want to monitor your methylmalonic acid levels to determine if your vitamin B-12 deficiency is progressing. Your doctor may also order additional tests to determine the cause of the deficiency. These tests include homocysteine and folate.
High levels of methylmalonic acid 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 kidneys can prevent the filtering of methylmalonic acid from the blood. This can result in increased levels of methylmalonic acid in the bloodstream. Therefore, the presence of high methylmalonic acid levels in patients with kidney disease may not indicate the presence of a vitamin B-12 deficiency. The results of the test must be discussed with your doctor and evaluated along with other diagnostic tests to determine if a vitamin B-12 deficiency is present.