A biochemical perspective on the use of tandem mass spectrometry for newborn screening and clinical testing

Abstract

The first newborn screen was a clinical test to detect a disorder of the biochemistry of the amino acid, phenylalanine. This disorder, known as phenylketonuria, produces profound mental retardation if not detected and treated early in life. Early screening programs relied on inexpensive population screening techniques that have all but been replaced by more accurate analytical methods such as tandem mass spectrometry (MS/MS). MS/MS enables a multianalyte approach for detecting biochemical disorders such that a metabolic profile is obtained rather than a single analyte measurement. The metabolic profile has clearly shown improvements in the detection of diseases such as phenylketonuria and several new disorders arising from errors in fatty acid oxidation and organic acid metabolism. MS/MS is a powerful tool for accessing the metabolic status of a newborn and can detect both inborn metabolic errors as well as examine the effect of acquired diseases or pharmacologic intervention on intermediary metabolism.

Introduction

Blood was obtained at 28 h of age from two newborns, infant A and infant B. A report from the newborn screen of their dried blood samples included a quantitative phenylalanine (Phe) of 240 μmol/L and 360 μmol/L for patients A and B, respectively. These results were considered presumptive for phenylketonuria (PKU), as they were greater than a laboratory defined normal cutoff of 180 μmol/L. A second specimen was obtained from these infants at 8 days of age. The result of these 2nd specimens showed Phe concentrations of 720 μmol/L and 80 μmol/L for infants A and B, respectively. Infant A was seen by a metabolic specialist and sent to a PKU treatment center for further evaluation and testing, resulting in confirmation of PKU and initiation of dietary management. Infant B had a normal Phe concentration and was therefore considered normal with no further follow-up required. The results for Infants A and B were recorded as true positive and a false positive, respectively.

This scenario is often encountered in newborn screening laboratories where the numbers of true and false PKU results are affected by the established criteria for what is a considered a “normal” phenylalanine concentration alone. One question that should be asked is what produced the initial abnormal Phe in patient B? What could have been done differently to avoid reporting this patient as presumptive for PKU? Examination of the biochemistry of Phe can improve the understanding and characterization of alterations in the metabolism of Phe, leading to better use of clinical methods, improved reporting of results, and a more timely and cost-effective medical care.

This review will focus on how tandem mass spectrometry (MS/MS) can be used to improve clinical analysis for disorders such as PKU and how it also can be used to expand and improve newborn screening for other inherited metabolic problems. Unlike manuscripts pertaining to this MS/MS application that we and others have published [1], [2], the focus of this review will be on the biochemistry and subsequent analysis of some key diseases that are a subset of the panel that can be screened using MS/MS. This review will also demonstrate that the choice of a metabolic screening approach that examines multiple metabolites is more likely to better characterize a particular metabolic disease or other iatrogenic influence such as total parenteral nutrition than a one-method, one-metabolite, one-disease approach. Three metabolic diseases will be described that represent disorders of amino acid, fatty acid, and organic acid metabolism. Emphasis will be placed on the laboratory result and how it can be interpreted clinically and better utilized in the process of diagnosing a disorder, integrating biochemical pathways where appropriate. Clinical situations typically encountered in a newborn screening laboratory will be used as illustrations.