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Cсылки по проблемам Акушерства и Геникологии
СТУДЕНТ ----- ЯКОВУ! Maternal Serum Alpha-Fetoprotein and Other Serum Markers BACKGROUND: Maternal serum measurement of alpha-fetoprotein AFP, human chorionic gonadotropin HCG and unconjugated estriol UE3 are used as screening tools to detect fetuses with neural tube defects anencephaly, encephalocele, meningomyelocele, ventral wall defects omphalocele, gastroschisis as well as several cytogenetic trisomies. Whereas measurement of AFP alone in conjunction with maternal age may detect up to 20% of fetuses with trisomy 21 Down syndrome, measurement of all three markers can increase the detection rate to 60%. AFP is a glycoprotein synthesized in the fetal GI tract and liver and excreted in the fetal urine and amniotic fluid AF. In amniotic fluid, AFP reaches its peak concentration between the 15th and 16th week of gestation and can be measured in both the AF and maternal serum MS by immunoassay. AFP may be elevated in pregnancies with neural tube defects and low in pregnancies with Down syndrome. HCG is a glycoprotein produced by the developing placenta and may be elevated in trisomy 21 pregnancies. UE3 is produced by the fetus and placenta and may be low in fetuses with intrauterine growth retardation or in a failing pregnancy. INDICATIONS FOR TESTING: Any pregnant woman may benefit from serum testing if they wish to revise their a priori risks for having a child with neural tube defects or chromosomal trisomies. It is especially useful for women under 35 years of age who would not be considering amniocentesis for advancing maternal age or for a woman with no family or pregnany history of problems warranting additional testing. PROCEDURE: A serum sample should be drawn ideally between 15 and 18 weeks gestation for best predictive results. High AFP: For the 3.5% of women who have AFP values above 2.0 multiples of the mean MOM a repeat sample is recommended if the pregnancy is not too far advanced. About half of those retested will have a normal second value and require no further testing. For those who have a second elevated level, a fetal sonogram should be performed to confirm gestational age and detect a multiple gestation since inaccurate dating and more than one fetus may significantly alter the predicted normal values. In some cases, a targeted ultrasound may reveal the presence of a neural tube or ventral wall defect in the fetus. For women whose gestational age is confirmed and there is only a single fetus, recalculation of the AFP is not necessary and these patients should be offered amniocentesis for amniotic fluid AFP determination and a level II sonogram. Low AFP: In some centers AFP alone is used as a preliminary screening test for Down syndrome risk assessment. However, it is clear that the measurement of AFP, HCG and UE3 is a better predictor of this risk. The risk analysis factors in maternal age, maternal weight and MOMs for AFP, HCG, and UE3. A risk of 1 in 270 or less is considered a positive screen since this is the equivalent numerical risk for a woman at 35 years of age. Under no circumstances should a low AFP or triple screen be repeated since a second normal value has not been shown to modify the initial risk. If there is a discrepancy, clarification of gestational age with recalculation on a first sample is warranted. Use of the triple screen to modify the risks of women at age 35 or older has certain inherent limitations since serum screening may not detect other chromosomal trisomies for which the risk is also increased. At present a separate screening profile is used to identify women who are at increased risk for having a fetus with trisomy 18. Low levels of AFP, HCG and UE3 at 0.75 MOM, 0.55 MOM and 0.60 MOM respectively are used to target such pregnancies. SAMPLE REQUIREMENTS: Between 15 and 18 weeks gestation 5cc of serum should be obtained in an anticoagulant free tube, preferably with a serum separator. A completed questionnaire must accompany each sample since serum concentrations change dramatically during the second trimester and correct interpretation of results relies heavily on maternal age, gestational age and maternal weight. INTERPRETATION: Risks for neural tube defects, other defects, trisomy 21 and trisomy 18 will be calculated based upon maternal age, weight and gestational age. If the actual gestational age is found to be discrepant by 4 weeks or more from the gestational age based on LMP, recalculation of the risk is recommended. For additional advice on what to do with an abnormal result please refer to the Procedure section. COUNSELING ISSUES: For patients with abnormal screening tests and a confirmed gestational age, genetic counseling is recommended. Prior to deciding upon a course of action, patients must understand the lab results, risks, limitations, advantages and disadvantages of various options. Interpreting the complexities of such results is difficult enough for the trained professional and is, at times, overwhelming to the pregnant woman. Genetic counseling may enable a patient to make rational decisions after fully discussing the laboratory findings. -------------------------------------------------------------------------------- Copyright 1999 by The Center for Human and Molecular Genetics CHMG/University of Medicine & Dentistry of NJ UMDNJ. The publisher disclaims any liability arising directly or indirectly from the use of this page and advises users to exercise their own informed judgment in the selection of medical care. Please review our full disclaimer.
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31.08.2000, 05:24
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