strates that cardiac decompensation can be avoided in MCD-deficient patients. Finally, by analogy with fatty acid oxidation disorders such as medium-chain acyl-CoA dehydrogenase deficiency (13 ), one may conclude that knowledge of the diagnosis will improve outcome because early intervention is then possible in catabolic episodes that otherwise may lead to metabolic derangement. It therefore seems reasonable to suppose that early diagnosis and treatment can reduce the morbidity and mortality associated with MCD deficiency and to include MCD deficiency in neonatal metabolic screening programs. In this study, the malonylcarnitine concentration in the neonatal screening test card was increased in only one of the two patients investigated, but this was a problem of stability after sample storage for several years.We conclude that (a) malonylcarnitine is detectable by ESI-MS/MS, (b) malonylcarnitine is increased in the blood of patients with malonic aciduria, and (c) ESI-MS/MSbased neonatal screening programs should be able to detect patients with MCD deficiency, a treatable metabolic disorder, before the development of symptoms. (1-5 ). Depending on the pH of the buffer, the activity of LDH can be measured by the increase as well as the decrease in NADH. The optimum pH for the pyruvate-to-lactate conversion is 7.4 -7.8. The German (2 ) and French (5 ) Societies for Clinical Chemistry recommend this reaction. The IFCC (4 ) recommends the lactate-to-pyruvate conversion, for which the optimum pH is 8.8 -9.8.In our laboratory, we have measured LDH according to the recommendations of the French society (SFBC) for years. This measurement was routinely performed with a Modular analyzer (Roche GmbH). When this SFBC method for LDH measurement needed to be replaced because our supplier stopped producing the necessary reagents, we decided to introduce the method based on the recommendations of the IFCC. Because we did not want to change our reference values (Ͻ450 U/L), we introduced a conversion factor to make the results obtained with the IFCC method directly comparable to the results obtained with the SFBC method.When we compared both methods using patient samples for which LDH was requested, we found numerous outliers, which forced us to do some additional investigations. To determine which method was responsible for the outliers, we began measuring duplicates for LDH with both methods. For these measurements we used lithiumheparin-plasma samples collected in plastic tubes with plasma separator (product no. 367994; Becton Dickinson). All measurements took place in the primary tubes, i.e., LDH measurements were performed directly in the lithium-heparin plasma after plasma was separated from the blood cells by centrifugation (1800g for 5 min), but without pipetting the plasma to secondary tubes. In these experiments, we found a rather high frequency of duplicate errors for LDH measurements with the IFCC method (27 of 152; 17.8%), whereas we found no such excessive frequency of duplicate errors with the SFBC method (2 of 140;...