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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 Causes of Depressed Maternal Serum Alpha-Fetoprotein Levels Neural tube defects (NTD) 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. 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. 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). Valid measurements of AFP in maternal serum cannot be made after amniocentesis. Triplet and higher multiple pregnancies cannot be interpreted. 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. 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.