These LC-MS-MS assays are routinely run in which acylcarnitines are identified ac Cx, where x is the number of carbons in the acyl group. Samples are spiked with unlabeled and labeled internal standards. The mass isotopomer distribution of each peak is determined to characterize its labeling pattern.

This test, coupled with the assay of the profile of urinary organic acids helps in the characterization of a number of metabolic defects, such as inborn errors of fatty acid oxidation disorders.