To the Editor.
—In a recent Questions and Answers section, Dr Upshaw1 asked which test to use to establish true functional vitamin B12 deficiency. Dr Carmel, in reply, suggested the serum methylmalonic acid (MMA) assay noting, however, its limitations of expense and specificity.Recently, a review article on this subject recommended the urinary MMA test by gas chromatography mass spectrometry.2 Physicians should be aware of the advantages of the urinary MMA assay.3 The test is noninvasive and requires only a random spot urine specimen. Unlike serum MMA results, urinary MMA values are normalized to creatinine levels correcting for renal insufficiency and dehydration. The urine test is available for $50, which is considerably less expensive than the serum assay.The urinary MMA test by gas chromatography mass spectrometry provides physicians with a convenient means to detect vitamin B12—deficient individuals early, allowing intervention to prevent permanent
Norman EJ. Urinary Methylmalonic Acid to Detect Vitamin B12 Deficiency. JAMA. 1995;273(18):1420. doi:10.1001/jama.1995.03520420032026