|
Alpha-Fetoprotein Assay (AF-AFP) -
Alpha-fetoprotein (AFP) is a protein produced by the fetal
liver. The function of the protein is unknown. It is present in high
concentration in fetal blood and in lower concentration in fetal urine. AFP
is present in amniotic fluid which is composed primarily of fetal urine.
Amniotic fluid AFP (AFAFP) may be elevated in open fetal body wall defects,
most commonly open neural tube defects (NTD) and open ventral wall defects,
due to transudation from exposed fetal vessels and tissue. AFAFP is elevated
in congenital nephrosis from increased glomerular filtration of this
relatively small protein. In cases with elevated AFAFP, the risk for an open
NTD or other fetal abnormality depends on the degree of elevation in the
AFAFP MoM (multiple of the median), the results of amniotic fluid
acetylcholinesterase testing and other significant risk factors.
Specimen requirements:
Amniotic fluid, 2-5 ml, obtained at 14-24 weeks
gestation.
Transport:
Transport amniotic fluid samples at ambient temperature by
same day or overnight courier. In high temperatures, the specimen
should be packed with a refrigerant and shipped at 2-8 degrees C. If
shipment is delayed, freeze the sample at -20 degrees C and transport
on dry ice. This is necessary to prevent the degradation of alpha-fetoprotein.
Elevated temperatures and aging of the sample accelerate the
degradation of the protein. Amniotic fluid samples for chromosome
analysis should not be frozen.
Standard of Analysis: Amniotic fluid alpha-fetoprotein
is measured in micrograms/ml (µg/ml) by a solid phase,
two-site fluoroimmunometric assay on the Wallac AutoDELFIA system.
In normal pregnancies, the AFP concentration in amniotic fluid
decreases by about 10% with each gestational week during the
second trimester. Conversion of the AFP concentration to multiples
of the median (MoM) allows use of a single cut off level for
all gestational weeks. AFAFP values greater than or equal to
2.0 MoM are considered elevated.
Time required: Approximately one to two days is required
for AFAFP analysis.
Report: A written report is mailed to the referring
physician. Results for any samples with elevated AFAFP MoM or
at increased risk for NTD or other fetal abnormalities are telephoned
and/or sent by FAX to the physician's office followed by a written
report. In addition, results may be sent by email if requested.
CPT Code: 82106
Acetylcholinesterase (AChE) Assay
- Acetylcholinesterase (AChE) is a neural enzyme present in cerebral spinal
fluid and fetal blood. AChE is not present in maternal blood and is not
normally detectable in amniotic fluid. The abnormal presence of
acetylcholinesterase in amniotic fluid is suggestive of an open fetal
defect. When AChE is detected, the ratio of AChE to pseudocholinesterase (PChE),
a non-specific cholinesterase normally found in amniotic fluid, may help
distinguish open neural tube defects from open ventral wall defects or fetal
blood contaminated fluid.
Specimen requirements: Amniotic fluid, 2-5 ml, obtained from 14
to 24 weeks gestation. The gel electrophoresis may be performed
prior to 14 weeks or after 24, but
results are less reliable.
Transport: Transport amniotic fluid samples at ambient temperature by same
day or overnight courier. In high temperatures, the specimen
should be packed with a refrigerant and shipped at 2-8 degrees C. If
shipment is delayed, freeze the sample at -20 degrees C and transport
on dry ice. Amniotic fluid samples for chromosome analysis should
not be frozen.
Standard of analysis: Acetylcholinesterase is assayed in all
amniotic fluid samples for which there is an increased risk of a neural
tube defect or other fetal abnormality. Amniotic fluid is examined
for the presence of acetylcholinesterase and pseudocholinesterase
by using slab gel electrophoresis. When acetylcholinesterase is detected,
the ratio of acetylcholinesterase to pseudocholinesterase may help distinguish
an open neural tube
defect from an open ventral wall defect or fetal blood contaminated fluid.
Time required: Approximately one week is required for
the AChE assay.
Report: Results from any samples with detectable AChE will be telephoned
and/or sent by FAX to the physician's office. Written reports
will be mailed to the physician.
CPT Code: 82013
|
