Pilot Experience with an External Quality Assurance Scheme for Acylcarnitines in Plasma/Serum.

The analysis of acylcarnitines (AC) in plasma/serum is established as a useful test for the biochemical diagnosis and the monitoring of treatment of organic acidurias and fatty acid oxidation defects. External quality assurance (EQA) for qualitative and quantitative AC is offered by ERNDIM and CDC in dried blood spots but not in plasma/serum samples. A pilot interlaboratory comparison between 14 European laboratories was performed over 3 years using serum/plasma samples from patients with an established diagnosis of an organic aciduria or fatty acid oxidation defect. Twenty-three different samples with a short clinical description were circulated. Participants were asked to specify the method used to analyze diagnostic AC, to give quantitative data for diagnostic AC with the corresponding reference values, possible diagnosis, and advice for further investigations.Although the reference and pathological concentrations of AC varied among laboratories, elevated marker AC for propionic acidemia, isovaleric acidemia, medium-chain acyl-CoA dehydrogenase, very long-chain acyl-CoA dehydrogenase, and multiple acyl-CoA dehydrogenase deficiencies were correctly identified by all participants allowing the diagnosis of these diseases. Conversely, the increased concentrations of dicarboxylic AC were not always identified, and therefore the correct diagnosis was not reach by some participants, as exemplified in cases of malonic aciduria and 3-hydroxy-3-methylglutaryl-CoA lyase deficiency. Misinterpretation occurred in those laboratories that used multiple-reaction monitoring acquisition mode, did not derivatize, or did not separate isomers. However, some of these laboratories suggested further analyses to clarify the diagnosis.This pilot experience highlights the importance of an EQA scheme for AC in plasma.

Ataxia associated with increased plasma concentrations of pristanic acid, phytanic acid and C27 bile acids but normal fibroblast branched-chain fatty acid oxidation.

Investigations of peroxisomal function were undertaken in an 8-year-old girl who developed motor difficulties at the age of 3.5 years and went on to develop a progressive ataxia and dysarthria. There were no other neurological abnormalities and she was of normal intelligence. Analysis of plasma very long-chain fatty acids revealed a normal C26 concentration and normal C24/C22 and C26/C22 ratios. Analysis of branched-chain fatty acids showed an elevated plasma phytanic acid concentration of 60 mumol/L (normal < 15) and a considerably elevated pristanic acid concentration of 50 mumol/L (normal < 2). Plasma concentrations of the C27 bile acids 3 alpha, 7 alpha-dihydroxycholestanoic acid (DHCA) and 3 alpha, 7 alpha, 12 alpha-trihydroxycholestanoic acid (THCA) and of the C29-dicarboxylic acid were also increased. We postulated that these results might be due to deficiency of the peroxisomal branched-chain acyl-CoA oxidase, but when oxidation of branched-chain fatty acids was studied in cultured skin fibroblasts it was found to be normal. Alternative explanations for the accumulation of branched-chain substrates for peroxisomal beta-oxidation are discussed. Treatment with a low-phytanic acid diet arrested the progression of the ataxia and led to a slight improvement.

Phenylalanine in whole blood and plasma--a candidate reference method.

A method for the quantification of phenylalanine in whole blood and plasma by isotope dilution gas chromatography-mass spectrometry is presented. The use of an uncommon derivative allows a simple extraction procedure and the most basic mass spectrometer. The relative standard deviation was found to be 1.3% within-batch and 2.1% between-batch under optimum conditions, and the detection limit was found to be 7 pmol injected. The ruggedness of the procedure and sample handling conditions were also examined.

Ratios for very-long-chain fatty acids in plasma of subjects with peroxisomal disorders, as determined by HPLC and validated by gas chromatography-mass spectrometry.

We describe an HPLC method for measurement of ratios of concentrations of very-long-chain fatty acids (VLCFA) in plasma. The method, which involves ultraviolet detection of p-bromophenacyl derivatives of fatty acids, is validated by comparison with a gas chromatographic-mass spectrometric (GC-MS) method. The correlation between the ratios of 24-carbon fatty acids to 22-carbon fatty acids (C24/C22) estimated by the two methods was close (r = 0.976) as was the correlation for the C26/C22 ratios (r = 0.947). Increased VLCFA ratios could be demonstrated by either technique in patients with adrenoleukodystrophy, Zellweger syndrome, and infantile Refsum's disease. The HPLC method also measures phytanate concentrations in plasma. Control VLCFA ratios (for subjects without peroxisomal disorders) obtained by the two methods agree well with those reported by Moser et al. (Ann Neurol 1984; 16:628-41). For subjects younger than one year, ratios for C24/C22 and C26/C22 fatty acids were significantly greater than in older subjects.