Biological variation of methylmalonic acid in urine in spanish population

Methylmalonic acid (MMA) is currently the best biomarker of functional vitamin B12 deficiency. However, for a correct interpretation of the patient's results it is necessary to know its biological variation (BV). No BV data are available for urine MMA values, as measured by mass spectrometry. Hence, the aim of this study was to estimate the within- and between-person coefficients of variation (CV_w, CV_g) for MMA in a healthy population, and the associated index of individuality (II), as well as to define quality specifications based on BV and the reference change value (RCV).

Random urine samples from 34 healthy volunteers were collected over four consecutive weeks. Samples were stored at -80 °C until analysis in a single analytical run. MMA excretion was quantified by tandem liquid chromatography coupled to mass spectrometry (HPLC-MS/MS). Results were normalized to urine creatinine. The coefficients of variation were estimated by CV-ANOVA. Confidence intervals (95 %) were calculated. Quality specifications were defined according to international recommendations.

A total of 128 samples were included. The coefficients of variation were CV_w = 35.7 % (26.1-45.3) and CV_g = 67.7 % (58.3-77.0). The associated II was 0.5 and the RCV was 88.1 %.

Considering the II obtained, MMA in urine has high individuality, therefore, RCV is better to evaluate serial clinical results. Our results will contribute to a better clinical interpretation of this biomarker and will represent a great aid when defining analytical performance specifications for this magnitude.