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AFP, Triple Screen, Quad Screen and Penta Screen Testing

Genetic Screening for Neural Tube Defects and Down Syndrome

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Blood draw in lab
Photo © iStockPhoto

Genetic testing, of any sort, usually brings a large debate. The alphfetoprotein (AFP), also known as the triple screen, quad sreen or penta screen, are no exceptions.

The AFP test, sometimes called the MSAFP or maternal serum AFP, is where a blood sample is drawn from the mother to check the levels of AFP. AFP is a protein secreted by the fetal liver and excreted in the mother's blood. It is generally used for detecting neural tube defects, but it can also indicate: abdominal wall defects esophageal and duodenal atresia, some renal and urinary tract anomalies turner syndrome, some low birth weight fetuses, and placental complications. A low level of AFP could also indicate Down Syndrome.

The triple screen measures not only AFP, but beta-hCG and unconjugated estriol (uE3) as well. This test is more accurate and screens for additional genetic problems, and is beginning to replace the standard AFP. Generally speaking, any combination of the testing will identify 60% of the babies with Down Syndrome and 80-90% of the babies with neural tube defects.

In addition to the substances screened in the triple screen, the quad screen checks the level of the hormone inhibin A as well. The penta screen looks for those four substances as well as Invasive Trophoblast Antigen (ITA). Which screening test you have will depend on which lab is available to you. If you also had the first trimester screening along with the nuchal fold translucency ultrasounds, it is recommended that you use the same lab.

The controversy in these tests lies in many places. One of which is the accuracy of the screenings. While some claim that they have only a 5% "false positive" rate, most research finds that there are 80+% rate of positive tests while the baby is unaffected. The false positive rates depend on many factors, including appropriate gestational age, maternal age, weight and the presence of diabetes.

False positives may not seem like a problem, however, there are follow up tests that are done that do carry risks, like amniocentesis or chorionic villus sampling (CVS), which does carry a 1 - 2% rate of fetal loss. The standard is usually to repeat the AFP or triple screen. Another positive screen will then go to the Level II Ultrasound. Ultrasound can even be used to detect some of the anomalies associated with Downs Syndrome. If the ultrasound doesn't find a mix up in gestational age, or a multiple pregnancy, an amniocentesis is generally the next step.

There is a possible benefit of identifying neural tube defects before birth, because that can alter care. Such as the mode of delivery. It is generally considered safer for a baby with spina bifida to have a cesarean delivery. Although this can also be identified by ultrasound alone.

Of the positive test results, 90% of these babies will not have any anomalies. There may be other benign reasons for the elevated or low levels of the hormones, specifically the gestational age, or multiple pregnancies. The AFP test is generally done between the 16th and 18th weeks of pregnancy, while the triple screen can be done a bit earlier. The gestational age is very important, because the amount of hormone in your blood will vary with gestational age.

The American College of Obstetricians and Gynecologists have stated that they believe this test should be offered to all pregnant women, regardless of maternal age. However, many women are made to believe the testing is mandatory. Many who refuse are asked to sign waivers in order to not take the test. While others are finding that their insurance coverage has a part to play in the whole scenario.

The test results are generally given as either positive/negative or a risk ranking, such as: 1/47 chance of having a baby with X anomaly. As well as low/high rankings. It is important to remember that being at an increased risk does not mean your baby has the defect. You should also ask that your practitioner explain the results in detail to you.

Given the fact that amniocentesis and CVS have the ability to cause a pregnancy loss, some women will choose not to use these invasive tests because it may be more likely that they will cause harm to the pregnancy than to have a baby with an anomaly.

Whether or not to have the test is a very hard decision and a very personal one. Some women feel that no matter what is wrong that they would not terminate the pregnancy, and therefore do not want the test. While others are very happy to have the normal results, even knowing that there are no guarantees.

Sources:

Hyperglycosylated human chorionic gonadotropin (invasive trophoblast antigen) immunoassay: A new basis for gestational Down syndrome screening. Clin Chem. 1999 Dec;45(12):2109-19.

Integrated serum screening for Down syndrome in primary obstetric practice. Prenat Diagn. 2005 Dec;25(12):1162-7.

New Robin J.R. Blatt, Prenatal Tests, England Journal of Medicine 323, No. 9, Aug 30, 1990.

Reproducibility of risk figures in 2nd-trimester maternal serum screening for down syndrome: comparison of 2 laboratories. Clin Chem. 2006 Nov;52(11):2087-94. Epub 2006 Sep 21.

Benn PA, Makowski GS, Egan JF, Wright D.

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