DAVID B. MATCHAR, M.D.; JOHN R. FEUSSNER, M.D.; DAVID S. MILLINGTON, Ph.D.; ROBERT H. WILKINSON, M.D.; DOUGLAS J. WATSON, B.S.; DIANE GALE, B.S.
MATCHAR DB, FEUSSNER JR, MILLINGTON DS, WILKINSON RH, WATSON DJ, GALE D. Isotope-Dilution Assay for Urinary Methylmalonic Acid in the Diagnosis of Vitamin B12 Deficiency: A Prospective Clinical Evaluation. Ann Intern Med. 1987;106:707-710. doi: 10.7326/0003-4819-106-5-707
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Published: Ann Intern Med. 1987;106(5):707-710.
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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