INTRODUCTION: Blood acylcarnitine profile analysis is a powerful tool to diagnose numerous inherited metabolic disorders, including many mitochondrial fatty acid oxidation disorders and organic acidemias. It is used for follow-up testing of screen positive results from newborn screening programs and in the evaluation of children and adults suspected of having a fatty acid or organic acid disorder. Serum or plasma samples, the latter obtained with a variety of anticoagulants, are normally accepted for acylcarnitine profile analysis. In view of the diverse types of blood collection tubes used in acylcarnitine analyses, it is important to evaluate possible matrix effects on the measurement of the many acylcarnitines that are assessed in blood acylcarnitine assays.
METHODS: More than 50 acylcarnitines and related analytes were measured by liquid chromatography-tandem mass spectrometry using plasma obtained from sodium heparin (green top tube), lithium heparin (green top tube), and EDTA (lavender top tube) anticoagulated tubes, as well as serum (red top tube). Samples from each blood tube type were obtained at the same time from three healthy individuals.
RESULTS: Several analytes showed significant (>30%) matrix effects. Among them, butyrobetaine was ~30-75% higher in heparinized plasma than in serum or EDTA anticoagulated plasma. Propionylcarnitine (C3) was about 35% lower when measured in serum than in all plasma tube types.
CONCLUSIONS: When differences of acylcarnitine concentrations between serum and plasma tubes straddle medical decision cut-offs, diagnostic decisions can be impacted; this may be particularly relevant for mild and late-onset forms of inherited metabolic disorders. These data suggest the utility of using matrix-specific reference intervals for those analytes that differ significantly between tube types.