Measurement of succinyl-carnitine and methylmalonyl-carnitine on dried blood spot by liquid chromatography-tandem mass spectrometry.

Methylmalonic aciduria (MMA) is one of the most frequent organic acidurias, a class of diseases caused by enzymatic defects mainly involved in the catabolism of branched-chain amino acids. Recently, mild MMA and C4-dicarboxylyl-carnitine (C4DC-C) accumulation have been reported in patients carrying mutation in genes encoding theα-subunit (SUCLG1) andtheβ-subunit (SUCLA2) of the ADP-forming succinyl-CoA synthetase (SCS). We developed a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method to quantify in dried blood spot the two isobaric compounds of C4DC-C, succinyl-carnitine and methylmalonyl-carnitine, to allow the differential diagnosis between classical MMA and SCS-related defects. Thismethod,with an easy liquid-phase extraction and derivatization procedure, has been validated to demonstrate the specificity, linearity, recovery, lowest limit of quantification (LLOQ), accuracy and precision for quantitative determination of blood succinyl-carnitine and methylmalonyl-carnitine. The assay was linear over a concentration range of 0.025–10 μmol/L and achieved the LLOQ of 0.025 μmol/L for both metabolites. The average slope, intercept, and coefficient of linear regression (r2) were respectively: 0.3389 (95% confidence interval 0.2888–0.3889), 0.0113 (95% confidence interval −0.0157 to 0.0384), 0.9995 (95% confidence interval 0.9990–1.0000) for succinyl-carnitine and 0.5699 (95% confidence interval 0.5263–0.6134), 0.0319 (95% confidence interval−0.0038 to 0.0677), 0.9997 (95% confidence interval 0.9995–1.0000) for methylmalonyl-carnitine. Withinday and between-day coefficients of variation (CV)were 1.94% and 3.19% for succinyl-carnitine and 3.21%, and 2.56 for methylmalonyl-carnitine. This method is accurate and provides a new tool to differentiate patients with classical methylmalonic acidemia from those with SCS-related defects.