Acetylcarnitine, C2

Laboratory assay overview

Elevation of one or more acylcarnitines is diagnostic for an organic aciduria or a fatty disorder. A low carnitine level may mask a disease state.

Laboratory assay overview

A plasma acylcarnitine profile is one of the most powerful and accurate methods to diagnose inborn errors of fatty acid metabolism and organic acidemias. In newborn screening, newborns have sufficient free carnitine provided through the placenta from the mother to make acylcarnitine profiling alone sufficient to rule out these inborn errors. Unfortunately, after the newborn period, at the time of presentation and diagnosis, many patients with these disorders have become chronically depleted of free carnitine due to the inability to recycle esterified carnitine. In this situation, the acylcarnitine profile may be normal with all values in the lower end of the normal range despite the patient being affected with a disorder of fatty acid oxidation or an organic acidemia. Recently, we had just such a case with a patient known to have methylmalonic acidemia because of organic acid analysis who initially had a normal acylcarnitine profile. In such cases, a carnitine analysis would have shown extremely low free carnitine and very elevated carnitine esters and would have prompted carnitine supplementation of the patient followed by further periodic evaluations. As clinicians come to rely increasingly on acylcarnitine profiling to diagnose inborn errors of metabolism, it will be important to assay both carnitine (free and esterified) simultaneously with acylcarnitines for the initial evaluation of children for inborn errors of metabolism. This approach would prevent false negative results and false reassurance that could occur if acylcarnitines alone were ordered.