Determination of total and free plasma carnitine concentrations on the Dade Behring Dimension RxL: integrated chemistry system.

BACKGROUND

L-carnitine is a naturally occurring quaternary ammonium compound present in all mammalian species. Its major function is to facilitate the passage of long-chain fatty acids through the mitochondrial membrane for subsequent beta-oxidation and ketone synthesis. Clinical interest in carnitine disorders relates particularly to possible deficiency states that may result in a phenotypic spectrum that includes cardiomyopathy, skeletal myopathy, hypoglycemia and hyperammonemia. The objective of this study was to develop a method on the Dade Behring Dimension RxL analyzer for measuring free and total carnitine levels in plasma.

METHODS

Plasma samples were deproteinized by ultrafiltration to remove interference by endogenous thiols. Filtrates were measured directly on the RxL for free carnitine or after alkaline hydrolysis for total carnitine by an endpoint enzymatic assay that uses carnitine acetyltransferase.

RESULTS

Within-run imprecision was <5% at high and low levels for both free and total carnitine while between-day imprecision was <15%. Recovery of free carnitine from spiked plasma >90%. The method was linear between 5.0 and 150.0 micromol/l and the limit of quantification was 5.0 micromol/l. Comparison of our method with another automated procedure developed on the Hitachi 917 system using Deming regression analysis resulted in the following equations: Dimension=1.034(Hitachi)-7.44 for total carnitine (r=0.955) and Dimension=0.805(Hitachi)+1.96 for free carnitine (r=0.951), respectively.

CONCLUSIONS

Our method is suitable for analyzer platforms where the level of imprecision is lower and the throughput is higher than manual methods. It also avoids the use of radioisotopes and is appropriate in labs where access to reference methods such as tandem mass spectrometry and HPLC is limited or unavailable.