Tandem Mass (MS)
Tandem mass spectrometry in newborn screening
Tandem mass spectrometry (MS/MS) has been used for severalyears 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.
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
1 More recently, MS/MS has been used in pilot
ions
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
procedure (e.g., fast atom bombardment or electrospray)
to create quasimolecular ions, and is injected into the first
quadrupole, which separates these
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
fragment to produce a particular daughter ion, or a
based on their mass/charge (m/z) ratios. In gas chromatography-
soft ionizationparent ions from each other.parent ion mode to obtain an array of all parent ions thatneutral lossmode to obtain an array of all parent ions that lose a common
neutral fragment. Further, these
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
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,
scan functions can be changeda-amino acids (by loss of a neutral 1022
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
guideline should not be considered inclusive of all proper procedures and tests or exclusive of other procedures and tests that are
reasonably directed toward obtaining the same results. In determining the propriety of any specific procedure or test, the geneticist
should apply his or her own professional judgment to the specific clinical circumstances presented by the individual patient or
specimen. It may be prudent, however, to document in the patient’s record the rationale for any significant deviation from this
guideline.C M G / A S H G s t a t e m e n t
Genetics
It is important to note that MS/MS cannot replace current
programs to screen for biotinidase deficiency, hypothyroidism,
hemoglobinopathies, virilizing adrenal hyperplasia, and
galactosemia; these conditions cannot be identified by MS/MS
at this time and must be detected by other means.
Several issues must be considered before MS/MS is added to
ongoing newborn screening programs. The instrument itself,
including the computer and autosampler, is expensive, access
to alternate instruments is imperative in the event of breakdown,
and laboratory personnel must be trained extensively to
operate and maintain it. Nonetheless, if the cost of instrumentation
is amortized over several years, MS/MS probably can be
added to existing newborn screening systems for an incremental
cost on the order of $20 per sample. It is important to note
that the cost of screening itself would be the same regardless of
the number of tests added to the screening menu. Costs for
other screening components, however, e.g., patient retrieval,
verification of diagnosis, treatment, etc., would vary.
The inclusion of additional disorders in the newborn
screening menu could increase the number of patients identified
each year by 50% to 100%, and more physicians, nutritionists,
and genetic counselors will be needed to deal with
their ongoing medical and nutritional care. Reimbursement
for the medical foods needed to treat these disorders must also
be addressed, because many third-party payers do not cover
medical foods, and state laws and regulations regarding reimbursement
vary.
It has been argued that MS/MS analysis should not be used
in newborn screening until more is known about its sensitivity
(false negatives) and specificity (false positives) for each of the
diagnosable disorders. Extensive experience with MS/MS, albeit
mostly with patients outside of the immediate newborn
period, has shown that the number of false positives is very
small. As was the case with all current screening methods, the
number of false negatives will only be learned after newborn
screening is implemented, and children that are not detected as
newborns are diagnosed later in life. Thus, as with all newborn
screening methods, screening should be accompanied by follow-
up sufficient to ensure that data on false negatives and
false positives is collected. These considerations argue for pilot
demonstration programs with adequate resources to acquire
and report technical and clinical results.
Many conditions that can be detected by MS/MS, such as
citrullinemia, propionic acidemia, and methylmalonic acidemia,
do not respond consistently to treatment. Nonetheless,
some patients do better with early diagnosis and treatment,
and early diagnosis can avoid trauma and expense to the family
and allow options for family planning to be considered before
other affected siblings are born.
The issue of informed consent for MS/MS screening is complicated,
in part because uniformly effective therapies have not
been developed for all the conditions the methodology can
detect and because it may detect previously unrecognized metabolites
and/or disorders. An example is the detection of
asymptomatic maternal 3-methylcrotonyl-CoA carboxylase
deficiency by acylcarnitine screening of newborn blood spots.
However, the computer parameters of the MS/MS can be set to
ignore certain molecular ions if a decision is made not to screen
for a particular disorder.
In summary, MS/MS can provide substantial benefits to patients
and their families if thoughtfully integrated into newborn
screening programs, provided that sufficient funding is
made available to cover the costs of the additional and necessary
personnel, medications, and medical foods. Indeed, the
expense and complexity of the instrumentation and the need
for trained metabolic physicians to care for the additional patients
could make it very difficult for states with small populations
and/or few trained personnel to implement MS/MS, and
development of regional laboratories and services may well be
necessary to address this need.
currently covered (Table 1).
Table 1
Some disorders detectable by tandem mass spectrometry
Disorder Diagnostic metabolite
Amino acidemias
Phenylketonuria Phenylalanine & tyrosine
Maple syrup urine disease Leucine
1 isoleucineHomocystinuria (CBS deficiency) Methionine
Citrullinemia Citrulline
Hepatorenal tyrosinemia Methionine & tyrosine
Organic acidemias
Propionic acidemia C
3 acylcarnitineMethylmalonic acidemia(s) C
3 acylcarnitineIsovaleric acidemia Isovalerylcarnitine
Isolated 3-methylcrotonylglycinemia 3-Hydroxyisovalerylcarnitine
Glutaric acidemia (type I) Glutarylcarnitine
Hydroxymethylglutaric acidemia Hydroxymethylglutarylcarnitine
Fatty acid oxidation disorders
SCAD deficiency C
4,6 acylcarnitinesMCAD deficiency C
8,10:1 acylcarnitinesVLCAD deficiency C
14,14:1,16,18 acylcarnitinesLCHAD and trifunctional protein
deficiency
C
14,14:1,16,18 acyl- and 3-hydroxyacylcarnitines
Glutaric acidemia type II Glutarylcarnitine
CPT-II deficiency C
14,14:1,16,16:1 acylcarnitines
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 withMCADdeficiency 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.
5–6 Among these are mediumchain
This guideline is designed primarily as an educational resource for medical geneticists and other health care providers to help them
provide quality medical genetic services. Adherence to this guideline does not necessarily ensure a successful medical outcome. This
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