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Biochemical Screening for the Detection of Down Syndrome

These tests are aimed at identifying within the population of pregnant women those who are at high risk of having an infant with Down syndrome and other congenital defects. They are screening rather than diagnostic tests. The difference between them is that the screening tests identify those women who should undergo further tests such as amniocentesis, chorionic villus sampling or special ultrasonography, which are diagnostic tests. These give an accurate evaluation about the condition of the fetus. They are usually invasive tests that carry a certain risk for the pregnancy, so that the purpose of the filtering (screening) tests is to identify those women who need to undergo an invasive test.

In general, the goals of the screening tests are to identify the 5% of pregnant women for whom amniocentesis is recommended. These women are often apprehensive about the test results, but in fact, the result is normal in approximately 98% of subjects.

What biochemical tests are available?

There are two biochemical tests that are used to evaluate the risk for Down syndrome. One is performed in the first trimester of pregnancy and the other in the second. These tests measure the levels of a number of substances secreted either by the fetus or the placenta, whose levels are significantly correlated with Down syndrome in certain weeks of pregnancy, as different quantities are secreted at different ages of gestation.

What may affect the results?

It is very important for the age of the pregnancy (gestational age) to be determined precisely.

The results are calculated according to the gestational age as determined by the ultrasound examination and according to the date of the mother's last period.

Also taken into account are such factors as the mother's weight, whether she suffers from any chronic illnesses such as diabetes mellitus, and whether the pregnancy is spontaneous or following fertility intervention.

Sometimes abnormal values do not result from a true pathological condition, but from imprecise assessment of the gestational age, or because the pregnancy started as a twin pregnancy where one of the fetuses later stopped developing.

Is the test reliable in a multiple gestation?

It is important to note that the tests, especially for evaluation of the risk for Down syndrome, are reliable and accurate for pregnancies involving a single fetus, but their predictive power for multiple gestations is much lower. Why? The test is performed using the mother's blood, meaning that even if one of the fetuses is abnormal, the test results may still be normal because of the results of the other fetus or fetuses. Therefore in the case of multiple gestations, it is not advisable to rely on these results, and nuchal translucency ultrasound tests should be performed at the correct time. This test examines each fetus individually, so that it can predict more accurately the risk of Down syndrome.

What is the significance of the test result?

The result of the screening tests is obtained as a weighted risk. One out of a number of women with an identical result will have a child with Down syndrome. Hence a woman receiving a result indicating a 1 in 100 risk will be at greater risk than a woman who has received a 1:500 result. In the former case, out of 100 women with identical results, 99 will have normal infants and one will have an affected one. In the latter case, 499 women with identical values will have normal children and one will have an affected one. A risk in excess of 1:386 is considered a high risk (a result defined as "abnormal"). These women are called for genetic counseling, where the results are explained to them and they are to undergo invasive diagnostic testing (amniocentesis or chorionic villus sampling, depending on the week of gestation). The result of the invasive test will give an exact, final answer as to whether the fetus has Down syndrome, another chromosomal disorder, or normal chromosomes.

Considering a risk of 1:386 as the borderline between high risk and lower risk means that the predictive power of the screening test is not 100% absolute, and there is a possibility that children with Down syndrome will be born to a certain percentage of women despite the result of the screening test having been regarded as "normal". On the other hand, there is a more common situation in which a woman receives an "abnormal" result in a screening test, only for the fetus to be found to be normal in prenatal testing (chorionic villus sampling or amniocentesis) performed subsequently. Essentially, approximately 98% of women who receive an abnormal result in the screening test have healthy fetuses. Despite these limitations, a risk group can be reliably defined using this test. Performing amniocentesis on women in the high-risk group has often prevented the birth of infants with chromosomal defects.

It should be noted that the biochemical screening tests can also indicate conditions apart from Down syndrome - for instance other chromosomal problems, defects of the closure of the spinal column, kidney defects, etc.

Biochemical screening test performed in the second trimester of pregnancy (the alpha-fetoprotein test)

This test is well known to most of the population and is routinely performed in all women between the 16th and the 18th weeks of pregnancy. It involves the testing of three markers: AFP, hCG and UE3. The levels obtained for the three markers, in conjunction with the mother's age, predict the risk for Down syndrome in 65% of cases. In addition, this test provides a weighted risk for trisomy 18 (the implication of the statistical risk obtained was described in the previous paragraph). Another advantage of this test is that an abnormal result for any of the individual markers is clinically significant irrespective of the weighted risk for Down syndrome.

The AFP marker. The normal range is 0.5-2.49 MoM (multiples of the mean). High levels of AFP (alpha-fetoprotein) indicate an increased risk of open defects of the central nervous system and the abdomen. Other problems, such as kidney diseases, defects of the integrity of the skin, defects in the closure of the abdominal wall, and placental defects, may also be indicated. When raised levels of this marker are found, the mother is advised to undergo a directed ultrasound test, and in certain cases amniocentesis too. In all these cases, the mother is invited to come for genetic counseling.

The hCG marker. The normal range is 0.2-3 MoM (multiples of the mean). High levels indicate a significantly raised risk for Down syndrome. Apart from this, high levels of hCG have no correlation with specific genetic problems. However, high hCG values can predict that the woman is at risk for developing pregnancy complications such as toxemia and placental problems. Usually, however, high values do not indicate a significant gynecological problem. On average, out of 100 women with high hCG values, only 10 will have obstetric complications, compared to 5 out of every 100 women with a normal hCG. It is important for the case physician to know these values when monitoring the fetus' growth and the mother's blood pressure.

The UE3 marker. The normal value is above 0.15 MoM (multiples of the mean). Extremely low levels (close to 0) indicate an increased risk for two genetic syndromes. One is a skin disease affecting males and causing fishlike scaly skin (ichthyosis), and the other is a syndrome causing many defects and mental retardation (SLO syndrome). There may also be other defects, so if low UE3 values are found, the mother is invited to come for genetic counseling and to undergo further tests.

Biochemical screening test in the first trimester for identifying women at high risk for Down syndrome

This is a newly introduced test, usually performed between the 9th and the 14th week of gestation, but preferably not later than the 12th week. In this test, the levels of two substances are measured - these are PAPA-A and hCG. The weighted results of these two markers in conjunction with the mother's age can predict the risk for having an infant with Down syndrome in 63% of cases. It is advisable to perform the test in conjunction with the fetal nuchal translucency ultrasound scan.
 
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Comments (6)


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