Download Alpha Fetoprotein Maternal Serum (MsAFP)

Alpha Fetoprotein Maternal Serum (MsAFP)- Single marker
screen
Alpha fetoprotein (AFP) is a single chain polypeptide which is similar in size and structure to
human serum albumin. In human embryos, AFP is first made in the yolk sac and later in the fetal
liver. As the fetal liver matures, it gradually switches to albumin synthesis. In the fetus, AFP
synthesis gradually declines to the normal adult range by 6 to 12 months after birth. AFP passes
by diffusion from the fetus through fetal glomeruli into the fetal urine and then into the
amniotic fluid. Amniotic levels peak in the first trimester and are 100 times lower than fetal
plasma levels.
AFP appears in maternal serum by transplacental transfer. During pregnancy, maternal serum
AFP levels rise steadily to a peak of approximately 500 ng/mL at about 32 weeks of gestation.
Thereafter, they decline until term.
AFP is thought to leak directly into the amniotic fluid causing unexpectedly high concentrations of
AFP. Subsequently, the AFP reaches the maternal circulation; thus producing elevated serum
levels. Other fetal abnormalities such as omphalocele, gastroschisis, congenital renal disease,
esophageal atresia, and other fetal distress situations such as threatened abortion and fetal
demise also may show AFP elevations. Increased maternal serum AFP values also may be seen
in multiple pregnancies and in unaffected singleton pregnancies in which the gestational age has
been underestimated.
Lower maternal serum AFP values have been associated with an increased risk for genetic
conditions such as trisomy 21 (Down syndrome) and trisomy 18. Risks for these syndrome
disorders are only provided with the use of multiple marker screening.
Measurement of maternal serum AFP values is a standard tool used in obstetrical care to identify
pregnancies that may have an increased risk for NTD.
Causes of Elevated Maternal Serum
Alpha-Fetoprotein Levels
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Causes of Depressed Maternal Serum
Alpha-Fetoprotein Levels
Neural tube defects (NTD)
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Gestational age younger than
calculated
Spina bifida
Anencephaly
Congenital skin defects
o Pilonidal cysts
o Abdominal wall defects
Gastrointestinal defects
Obstruction
Gestational age older than expected
Chromosomal trisomies
Hydatidiform mole
Fetal demise
Increased maternal weight
Neural tube defects (NTD), such as spina bifida and anencephaly, result from the incomplete
closure or functional incompetence of the neural tube. This results in leakage of fetal AFP into
the amniotic fluid and increased levels in maternal sera. Screening for NTD is most useful and
accurate when performed between 15 - 20 weeks gestation because there is a greater spread in
normal values with advancing pregnancy. A screening program detects approximately 88% of
fetuses with anencephaly and 77% of those with open spina bifida.
Increased levels are also seen with multiple pregnancy, fetal distress and intrauterine death.
Down syndrome
Decreased levels of AFP are associated with Down syndrome. Prenatal AFP screening detects
20% of affected fetuses in women less than 35 years of age. Abnormal AFP levels are not
diagnostic for neural tube defect or Down Syndrome, but define a population that requires
further testing. The reference range is 0.25 - 2.50 MoM (multiples of the median) and a
predicted risk for neural tube defect of Down Syndrome < 1/270.
Cautions
 Race, weight, multiple fetus pregnancy, and insulin-dependent diabetes (IDD) may affect
marker concentrations.
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The screen results are dependent on accurate information for gestation, race, insulin
dependent diabetes, and weight. Inaccurate information can lead to significant
alterations in the estimated risk. In particular, erroneous assessment of gestational age
can result in false-positive or false-negative screen results. Because of its increased
accuracy determination of gestational age by ultrasound is recommended, rather than
by maternal dates alone when possible.
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A screen-negative result does not guarantee the absence of fetal defects. A screenpositive result does not provide a diagnosis, but indicates that further diagnostic testing
should be considered (an unaffected fetus may have screen-positive result for unknown
reasons).
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Valid measurements of AFP in maternal serum cannot be made after amniocentesis.
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Triplet and higher multiple pregnancies cannot be interpreted.
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Each center offering maternal serum screening to patients should establish a standard
screening protocol, which provides pre- and post-screening education and appropriate
follow-up for screen-positive results.
Specimen Required
Container/Tube: Plain, red-top tube or serum gel tube
Specimen Volume: 1 mL of maternal serum
Second Trimester Maternal Screening Alpha-Fetoprotein (AFP)/QUAD Screen
Patient Information Sheet is required.
Collection Instructions: Immediately spin specimen down.
Do not draw specimen after amniocentesis, as this could affect results.
Reject Due To
Specimens other than
Anticoagulants other than
Hemolysis
Lipemia
Serum
NA
Mild OK; Gross reject
Mild OK; Gross OK
Persson PH, Kullander S, Gennser G, Grennert L, Laurell CB. Screening for fetal malformations
using ultrasound and measurements of alpha-fetoprotein in maternal serum. Br Med J (Clin Res
Ed). 1983 Mar 5;286 (6367):747-9.
Rose NC, Mennuti MT. Maternal serum screening for neural tube defects and fetal chromosome
abnormalities. West J Med. 1993 Sep;159(3):312-7. Review.
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Screening for NTD is most useful and accurate when performed between 15 - 20 weeks
gestation because there is a greater spread in normal values with advancing pregnancy.
Abnormal AFP levels are not diagnostic for neural tube defect or Down Syndrome, but
define a population that requires further testing.