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Tandem MS (MS/MS or tandem mass spectrometry) is a
technology that is used in various ways to measure many different
compounds. Specific tandem MS methods can detect elevated levels
of biochemical compounds in blood that are useful for diagnosing
many inherited disorders of metabolism. It is important to realize
that the different types of tandem MS tests are designed to detect
specific disorders and therefore may not diagnose some diseases.
Supplemental Newborn Screening is typically done in conjunction
with state newborn screening and may include other tests in addition
to tandem MS.To determine what your child may be tested for, one
should request an information sheet from your testing laboratory.
This should list what disorders will be tested for, such as amino
acid disorders, organic acid disorders, fatty acid oxidation disorders
or other types of disorders.
For patients with clinical symptoms or family histories
of inherited metabolic disorders, diagnostic tests for specific
disorders are much more appropriate than a general screen. The specific
tests, such as urine organic acids, plasma amino acids, plasma or
dried blood spot acylcarnitines should be determined and ordered
by clinicians based on the clinical symptoms.
Supplemental Newborn Screening by tandem MS: In our
laboratory, the Supplemental Newborn Screening is done by tandem
MS for amino acids and acylcarnitines. The test can detect more
than 30 inherited disorders of amino acid, organic acid and fatty
acid metabolism. It is an inexpensive screening test with dried
blood spots designed for screening healthy newborns at 2-3 days
of age. It is not recommended for the diagnosis of children with
clinical symptoms. Cut offs for normal levels of amino acids and
acylcarnitines in newborns are chosen to minimize false positives.
Approximately 1% shows some slight elevation of an amino acid or
acylcarnitine which causes us to request a repeat dried blood spot
card to see if the slight elevation is significant. Usually the
repeat is normal. The false negative rate is not known but is believed
to be very low. In order for this test to be inexpensive ($25),
it is highly automated. The quantitative results are analyzed automatically
by tandem MS computer programs for "normality" using cut offs established
for different groups of subjects. Although the screening is most
accurate for newborns it is still worthwhile to perform for older
infants if they were not screened in the newborn period.
Quantitative Acylcarnitine profile by tandem MS is
a more expensive test designed for the differential diagnosis of
fatty acid oxidation and organic acid disorders in children with
clinical symptoms or family histories suggestive of these disorders.
This can be performed on dried blood spots, plasma, postmortem blood
(or amniotic fluid with prior consultation). Because it is intended
for diagnostic evaluation, the cut offs used for acylcarnitines
are lower and every profile is examined by a technician for any
abnormalities in the mass spectrum, including peaks that are not
part of the quantitative analysis. Every profile is reviewed by
an experienced professional and reported with a professional interpretation.
The acylcarnitine profile does not include amino acid disorders.
Dr. Larry Sweetman, PhD and Staff
Institute of Metabolic Disease
Baylor University Medical Center
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