Methylmalonic acid measured in plasma and urine by stable-isotope dilutionand electrospray tandem mass spectrometry

Background: Liquid chromatography-tandem mass spectrometry (LC-MS/MS) with electrospray ionization is robust and allows accurate measurement of both l ow- and high-molecular weight components of complex mixtures. We developed a LC-MS/MS method for the analysis of methylmalonic acid (MMA), a biochemic al marker for inherited disorders of propionate metabolism and acquired vit amin B-12 deficiency. Methods: We added 1 nmol of the internal standard MMA-d(3) to 500 muL of pl asma or 100 muL of urine before solid-phase extraction. After elution with 18 mol/L formic acid, the eluate was evaporated, and butyl ester derivative s were prepared with 3 mol/L HCl in n-butanol at 65 degreesC for 15 min. Fo r separation, we used a Supelcosil LC-18, 33 x 4.6 mm column with 60:40 (by volume) acetonitrile:aqueous formic acid (1 g/L) as mobile phase. The tran sitions m/z 231 to m/z 119 and m/z 234 to m/z 122 were used in the selected reaction monitoring mode for MMA and MMA-d(3), respectively. The retention time of MMA was 2.2 min in a 3.0-min analysis, without interference of a p hysiologically more abundant isomer, succinic acid. Results: Daily calibrations between 0.25 and 8.33 nmol in 0.5 mt exhibited consistent linearity and reproducibility. At a plasma concentration of 0.12 mu mol/L, the signal-to-noise ratio for MMA was 40:1. The regression equat ion for our previous gas chromatography-mass spectrometry (GC-MS) method (y ) and the LC-MS/MS method (x) was: y = 1.030x - 0.032 (S-y/x = 1.03 mu mol/ L; n = 106; r = 0.994). Inter- and intraassay CVs were 3.8-8.5% and 1.3-3.4 %, respectively, at mean concentrations of 0.13, 0.25, 0.60, and 2.02 mu mo l/L. Mean recoveries of MMA added to plasma were 96.9% (0.25 mu mol/L), 96. 0% (0.60 mu mol/L), and 94.8% (2.02 mu mol/L). One MS/MS system used only o vernight (7.5 hi replaced two CC-MS systems (30 instrument-hours/day) to ru n 100-150 samples per day, with reductions of total cost (supplies plus equ ipment), personnel, and instrument time of 59%, 14%, and 75%, respectively. Conclusions: This method is well suited for large-scale MMA testing (greate r than or equal to 100 samples per day) where a shorter analytical time is highly desirable. Reagents are less expensive than the anion-exchange/cyclo hexanol-HCl method, and sample preparation of batches up to 100 specimens i s completed in less than 8 h and is automated. (C) 2000 American Associatio n for Clinical Chemistry.