First experience with a fully automated extraction system for simultaneous on-line direct tandem mass spectrometric analysis of amino acids and (acyl-)carnitines in a newborn screening setting

RATIONALE

For Newborn Screening (NBS) programs all over the world whole blood dried on filter paper, also referred to as dried blood spots (DBS), has been the standard specimen for decades. In recent years DBS have attracted the attention of pharmaceutical companies, mostly due to the low volume of collected sample and simplified, therefore more cost-efficient, transportation requirements. However, the classical NBS workflow did not totally fulfil the needs of their studies, especially with respect to high-throughput unassisted sample processing for tandem mass spectrometric (MS/MS) analysis. Automated on-line extraction systems for direct analysis have already been tested and proved to be suitable for these pharmaceutical applications.

METHODS

The suitability of the automated CAMAG DBS-MS 500 interface for simultaneous detection of amino acids and (acyl-)carnitines has been tested together with an Acquity TQD tandem mass spectrometer from Waters and MassChrom stable isotope labelled internal standards from Chromsystems. No chromatographic sample treatment was applied; instead, the extract was directly injected into the MS/MS instrument. The feasibility of the instrumental setting for the routine newborn screening was tested on original samples coming from previously diagnosed patients.

RESULTS

The performance of the automated extraction technique and its application in preliminary quantitative screening for amino acids and (acyl-)carnitines for NBS showed very promising results. Several samples from patients, each diagnosed with one of four different inborn errors of metabolism (IEM), were tested and the correlation with the conventional punch-and-elute approach was very good.

CONCLUSIONS

Although the presented method still needs further optimization, our study clearly shows the possibility to use direct on-line analysis in the NBS setting. Our report on direct on-line analysis of newborn samples is a first approach in the development of a fully automated screening method for NBS analysis. With regard to the chemical properties of the analytes, the study resulted in a readily applicable screening method.

Postprandial changes of amino acid and acylcarnitine concentrations in dried blood samples

Blood sampling for newborn screening cannot be standardized as for example blood collection in adults after an overnight fast. Therefore the influence of postprandial changes and individual variation is valuable information for the assessment of sensitivity and specificity of newborn screening for certain disorders. We have analyzed 92 pairs of dried blood samples taken pre- and one hour postprandially, respectively. We have determined the mean increase in metabolite concentration and calculated its significance. Individual variation after an overnight fast in healthy adults (n = 3) was between 12 and 32% (SD). Postprandial increases of acylcarnitines were mostly not significant and not exceeding 10%. Postprandial increase of amino acids was highly significant for most proteinogenic amino acids, but not for all. With the collected data we were able to estimate that mainly decreased levels of methionine and, to a lesser extent, of free carnitine could be "masked" by postprandial increases of the respective metabolites, and could therefore lead to false negative results for the detection of disorders of cobalamin metabolism and carnitine transporter deficiency.