Tandem mass spectrometry in newborn screening.

Tandem mass spectrometry (MS/MS) has been used for several years to identify and measure carnitine esters in blood and urine of children suspected of having inborn errors of metabolism. Indeed, acylcarnitine analysis is a better diagnostic test for disorders of fatty acid oxidation than organic acid analysis because it can often detect these conditions when the patient is not acutely ill.1 More recently, MS/MS has been used in pilot programs to screen newborns for these conditions and for disorders of amino and organic acid metabolism as well. The purpose of this article is to describe MS/MS and discuss its potential role in newborn screening programs. The mass spectrometer is a device that separates and quantifies ions based on their mass/charge (m/z) ratios. In gas chromatography-mass spectrometry of organic acids, for example, organic acid derivatives are first subjected to gas chromatography and then enter the mass spectrometer, where each is ionized and fragmented and the abundance and m/z ratio of the various fragment ions are determined. The modern tandem mass spectrometer usually consists of two quadrupole mass spectrometers separated by a reaction chamber or collision cell; the latter is often another quadrupole. The mixture to be analyzed is subjected to a soft ionization procedure (e.g., fast atom bombardment or electrospray) to create quasimolecular ions, and is injected into the first quadrupole, which separates these parent ions from each other. These ions then pass (in order of m/z ratio) into the reaction chamber, where they are fragmented; the m/z ratios of the fragments are then analyzed in the second quadrupole. Because separation of compounds in the mixture is by mass spectrometry instead of chromatography, the entire process, from ionization and sample injection to data acquisition by computer, takes only seconds. The computer data can be analyzed in several ways. One can use a parent ion mode to obtain an array of all parent ions that fragment to produce a particular daughter ion, or a neutral loss mode to obtain an array of all parent ions that lose a common neutral fragment. Further, these scan functions can be changed many times during analysis, so that one can detect and measure butyl esters of acylcarnitines (by the signature ion at m/z 85) and the butyl esters of a-amino acids (by loss of a neutral 102 fragment) in the same sample. MS/MS thus permits very rapid, sensitive and, with appropriate internal standards, accurate measurement of many different types of metabolites with minimal sample preparation and without prior chromatographic separation. Because many amino acidemias, organic acidemias, and disorders of fatty acid oxidation can be detected in 1 to 2 minutes, the system has adequate throughput to handle the large number of samples that are processed in newborn screening programs. Some conditions that can be diagnosed by MS/MS are listed in Table 1, together with the compound(s) on which diagnosis is based. Amino acid quantitation by MS/MS is more accurate than most methods now in use for newborn screening and would thus provide more specific and sensitive screening for phenylketonuria,2 maple syrup urine disease,3 and homocystinuria.4 Analysis by MS/MS would also permit the screening menu to be expanded to include a number of disorders that are not currently covered (Table 1).5– 6 Among these are mediumchain acyl-CoA dehydrogenase (MCAD) deficiency and glutaric acidemia type I (GA1), which are relatively common and difficult to detect before the onset of symptoms and whose outcome is substantially improved by early treatment. Infants with MCAD deficiency seem healthy in early infancy but develop episodes of hypoketotic hypoglycemia during the first years of life; the first episode is fatal in 30% to 50% of patients. Most of these deaths could be prevented if dietary treatment and measures to prevent fasting were begun before the onset of symptoms. Infants with GA1 develop normally until they suddenly develop acute encephalopathy and irreversible striatal damage during the first 2 to 3 years of life. There is increasing evidence that striatal damage can usually be prevented by L-carnitine and vigorous treatment of catabolic episodes if begun before the onset of symptoms.