Methylmalonic Acid

Test ID: 

703841

CPT code:

83921

Synonyms:

Diagnose cobalamin (vitamin B12) deficiency

Clinical Use:

Methotrexate is an antimetabolite that combines with dihydrofolate reductase and interferes with the synthesis of tetrahydrofolic acid necessary for purines, DNA synthesis. The half-life in plasma is only several hours, while the methotrexate polyglutamates persist in cells for weeks or months.

Test Information:

MMA is a four-carbon molecule that is a product of the metabolic break-down of valine, isoleucine, and propionic acid.1 Vitamin B12 is a critical cofactor for the conversion of MMA to succinate.1 As a result, vitamin B12 deficiency causes an accumulation of MMA in the serum.1 MMA concentrations will often become elevated in the early stages of vitamin B12 while serum vitamin B12 levels are in normal range.1-4 Consequently, MMA measurement is used as a diagnostic test for vitamin B12 deficiency in persons with a low or low normal serum vitamin B12 concentration.5,6 Follow-up measurement of MMA can also be of value in assessing the effectiveness of vitamin B12 supplementation of deficient patients.7

Vitamin B12 deficiency causes macrocytic anemias and decreased erythrocyte survival due to abnormal maturation of erythrocyte precursors in the bone marrow.10,11 Pernicious anemia is a form of vitamin B12 deficiency that is caused by a lack of intrinsic factor.10,11 Low vitamin B12 intake, gastrectomy, malabsorption, and transcobalamin deficiency can also cause vitamin B12 deficiency.10,11 Although severe vitamin B12 deficiency is associated with anemia, hematologic signs are not always observed in patients with biochemically confirmed deficiency.12 Elderly patients with cobalamin deficiency may present with peripheral neuropathy, ataxia, memory impairment, depression, and dementia in the absence of anemia.13,14

A generally agreed on cutoff for elevated plasma MMA is 370 nmol/L.6,12 Approximately 2% of the US population and 7% of elderly persons have MMA concentrations above this threshold.12

Specimen Type:

Serum or plasma

Requested Volume: 

2 mL

Minimum Volume: 

0.6 mL

Container Type: 

Red-top tube, gel-barrier tube, or green-top (heparin) tube

Patient Preparation: 

Collection:

Separate serum or plasma from cells within one hour of collection.

Storage Instructions:

Refrigerate.

Stability Requirements:

Rejection Criteria:

Plasma from a light blue-top or yellow-top tube; citrate interferes with assay.