, Melissa Yssel, MB ChB, FC Path(SA) Chem 139, and Wendy M. Zakowicz, BS 79 Purpose: To achieve clinical validation of cutoff values for newborn screening by tandem mass spectrometry through a worldwide collaborative effort. Methods: Cumulative percentiles of amino acids and acylcarnitines in dried blood spots of approximately 25-30 million normal newborns and 10,742 deidentified true positive cases are compared to assign clinical significance, which is achieved when the median of a disorder range is, and usually markedly outside, either the 99th or the 1st percentile of the normal population. The cutoff target ranges of analytes and ratios are then defined as the interval between selected percentiles of the two populations. When overlaps occur, adjustments are made to maximize sensitivity and specificity taking all available factors into consideration.

For best performance we suggest an IRT/PAP protocol with an IRT-cut-off close to the 99.0th percentile, FS, and a single PAP-cut-off.

Our results obtained in a large cohort of ∼330,000 newborns support the use of a purely biochemical IRT/PAP protocol as an acceptable alternative when genetic CF-NBS has to be avoided.

Electrospray-tandem mass spectrometry represents a powerful method for detection of inborn errors of fatty acid metabolism. In the present study, it was used to examine neonatal carnitine metabolism, which reflects fatty acid metabolism. In 70 healthy neonates, blood samples were taken from the umbilical cord and by heel-stick puncture in full-term neonates on postnatal d 5. Cord blood specimens were also obtained from 15 preterm and 10 small-for-gestational-age infants. Acylcarnitine concentrations were measured in dried blood spots by electrospray tandem mass spectrometry. Compared with cord blood, the levels of nearly all acylcarnitine species were significantly higher on the postnatal d 5, whereas free carnitine remained unchanged. Total acylcarnitine/free carnitine-ratio increased, whereas the free carnitine/total carnitine-ratio (0.54 Ϯ 0.05; p Ͻ 0.01) further decreased. A reduced availability of free carnitine in the early neonatal period may affect fatty acid oxidation and thus be of potential pathophysiological relevance under conditions with higher energy demands, e.g. in sepsis. Cord blood concentrations of free carnitine, total carnitine, and total acylcarnitines were strongly related to birth weight (p Ͻ 0.01). Lower umbilical artery pH, i.e. mild hypoxia, caused accumulation of mainly long-chain acylcarnitines. This implicates that long-chain acylcarnitines could serve as a parameter of perinatal asphyxia. In utero, the main substrates for fetal metabolism are amino acids and glucose. The human placenta is also permeable to FFA. However, they are mainly stored as triglycerides in adipose tissues and liver because of the constant supply of glucose and amino acids and a low capacity of fetal tissues for FFA oxidation. The cutoff of the nutrient supply in the presuckling period after birth leads to a transient period of starvation. Thus, the newborn infant is entirely dependent on the mobilization of glycogen and fat stores. With the initiation of milk feeding, neonatal metabolism switches to a high-fat diet. Therefore, it is obvious that fatty acid oxidation is very important in the early postnatal period (1).Carnitine plays an important role in oxidation of especially long-chain fatty acids. Carnitine palmitoyltransferases I and II mediate the transport of fatty acid-carnitine esters (acylcarnitines) across the mitochondrial membranes. Carnitine acyltransferases can also be found in peroxisomes (carnitine octanoyltransferase) and endoplasmatic reticulum (microsomal carnitine acyltransferase), although their function is not yet fully understood. In the mitochondrion, carnitine removes excess acyl groups and thus regulates the concentration of free CoA in the mitochondrial matrix (2). Several studies have shown that the carnitine pool in the neonate is limited (1, 3, 4). Tandem mass spectrometry allows quantitative determination of free carnitine and various carnitine esters like acylcarnitines (5, 6).In the present study, we used this technique to investigate the changes in neonatal carnitine and fat...