Objectives: We sought to determine the cutoffs of fasting urinary methylmalonic acid (MMA) indicating elevated and mildly elevated serum MMA concentrations in older persons.
Methods: We studied 113 female Chinese vegetarians older than 55 y with normal renal function. Fasting serum samples were obtained for measurement of vitamin B12, MMA, and folate and tests of renal function. A fasting urine sample was collected for MMA measurement by the stable-isotope dilution method. The correlation between serum and urinary MMA levels was examined. The optimal cutoffs of urinary MMA for predicting elevated and mildly elevated serum MMA were estimated by use of receiver operating characteristic curves.
Results: Fasting urinary and serum MMA levels were linearly correlated. Pearson's correlation coefficient was 0.94. The cutoff of fasting urinary MMA of 2 microM/mM of creatinine had a sensitivity of 79%, a specificity of 85%, and a positive predictive value of 93% for elevated serum MMA (> 0.4 microM/L). A cutoff of 1.5 microM/mM of creatinine had a sensitivity of 86%, a specificity of 85%, and a positive predictive value of 95% for mildly elevated serum MMA (> 0.3 microM/L). Both cutoffs had high positive predictive values for subnormal vitamin B12 concentrations.
Conclusion: Overnight fasting urinary MMA concentrations have a strong linear relation to serum MMA in older vegetarians without renal impairment. Urinary MMA is potentially useful as a screening tool for metabolic vitamin B12 deficiency in older persons.