Acylcarnitines, including in vitro loading tests

Acylcarnitine analysis for the diagnosis of organic acidemias and particularly of fatty acid oxidation (FAO) disorders plays an increasingly prominent role in all venues of clinical biochemical genetics: prenatal diagnosis, newborn screening, evaluation of symptomatic patients, and postmortem screening. Almost exclusively performed by tandem mass spectrometry (MS/MS), plasma/serum is the primary specimen type in diagnostic settings. Blood dried on filter paper is analyzed for newborn screening and together with bile in the postmortem evaluation of cases of sudden and unexpected death. Cell-free supernatant of amniotic fluid is used for the prenatal diagnosis of selected inborn errors of metabolism. Cultured fibroblasts or amniocytes can be probed with FAO substrates and carnitine. Cell cultures deficient of an FAO enzyme will accumulate specific acylcarnitine species when incubated with substrates such as palmitate, allowing for the diagnosis of FAO disorders and several organoacidopathies. Acylcarnitine analysis using stable isotope-labeled internal standards provides quantitative data for acylcarnitine species. However, to provide meaningful results to referring health care providers, it is critical to complement analytical proficiency with in-depth interpretation of results as is true for many other examples of complex metabolic profiles.