A simple method modification to increase separation of 2- and 3-hydroxyglutaric acid by GC-MS for clinical urine organic acids analysis.

Urine organic acids profiling by gas chromatography-mass spectrometry (GC-MS) is routinely performed in hospital biochemical genetics laboratories for the investigation of inborn errors of metabolism. In particular, accurate identification of urinary levels of 3-hydroxyglutaric acid (3-OHGA) is important for diagnosing glutaric aciduria type 1 (GA1), but can be challenging by routine GC-MS profiling analysis due to co-elution and spectral similarity with the isomer 2-hydroxyglutaric acid (2-OHGA). To improve analytical specificity, unique ions were selected and a simple second-tier reinjection method was developed to enhance the chromatographic separation of the 2- and 3-OHGA isomers and potential unknown interferences. Specimens flagging on the routine analysis were simply reinjected on the same GC column using a modified temperature gradient containing an isothermal hold. Correlation between the reinjection and initial methods was higher for 2-OHGA (R = 0.9612) compared to 3-OHGA (R = 0.7242). Mean differences between the reinjection and initial methods for 2-OHGA and 3-OHGA were -8.5% and -61.1% respectively. The large decrease in 3-OHGA concentration for many specimens using the reinjection method was primarily attributable to separation from unknown variable interference(s) that were falsely elevating 3-OHGA in the initial analysis despite the use of a more unique quantifier ion. Overall, the reinjection approach increased analytical specificity in evaluating for the presence of increased urinary 3-OHGA. This second-tier approach, using a GC isothermal hold, could easily be implemented or adapted by other clinical laboratories experiencing related diagnostic challenges.