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Isotope-Dilution Assay for Urinary Methylmalonic Acid in the Diagnosis of Vitamin B12 Deficiency: A Prospective Clinical Evaluation

DAVID B. MATCHAR, M.D.; JOHN R. FEUSSNER, M.D.; DAVID S. MILLINGTON, Ph.D.; ROBERT H. WILKINSON Jr, M.D.; DOUGLAS J. WATSON, B.S.; and DIANE GALE, B.S.
[+] Article and Author Information

Grant support: in part by grant 84F034 from the Veterans Administration Health Service Research and Development Service, and from the A. W. Mellon Foundation.

Presented in part at the 43rd annual meeting of the American Federation for Clinical Research, Washington, D.C., 5 May 1986 (Clin Res. 1986;34:376A).

▸Requests for reprints should be addressed to David B. Matchar, M.D.; 508 Fulton Street, Health Service Research Field Program (152), Durham VA Medical Center; Durham, NC 27705.


Durham, North Carolina


©1987 American College of PhysiciansAmerican College of Physicians


Ann Intern Med. 1987;106(5):707-710. doi:10.7326/0003-4819-106-5-707
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Vitamin B12 deficiency is a frequently considered diagnosis for which there is no single, commonly available and accurate test. A urinary methylmalonic acid assay using gas chromatography-mass spectrometry has been proposed as the preferred test. We reviewed vitamin B12 assays on 1599 consecutive patients and prospectively studied all patients with low serum B12 levels (n = 75) and a random sample of patients with normal levels (n = 68). Of 96 evaluable patients, 7 had clinical deficiency. All 7 deficient patients had urinary methylmalonic acid levels greater than 5 µ/mg creatinine (sensitivity, 100%; confidence interval, 65% to 100%). Of the 89 patients who were not clinically deficient, 88 had urinary methylmalonic acid levels less than or equal to 5 µ/mg creatinine (specificity, 99%). The overall test accuracy in this population was 99%. If the high sensitivity and specificity of the gas chromatography-mass spectrometry assay for urinary methylmalonic acid is supported by other clinical studies, the methylmalonic acid assay may become the reference standard for the diagnosis of vitamin B12 deficiency.

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