Screening Tests Used for Down Syndrome in Pregnancy
Screening tests are carried out to identify individuals with a high probability of any disease or anomaly within healthy population. When this aim is taken during pregnancy, it will mean that pregnancies with high probability of fetal anomaly can be detected. The purpose of screening tests during pregnancy is to determine the anomaly as early as possible and to inform the family about the situation.
Anomalies can be structural or chromosomal. For chromosomal diseases, we will talk about scanning tests in the blood combined with ultrasonography and invasive tests (amniocentesis, cordocentesis and chorionic villus biopsy) for the diagnosis of chromosomal diseases and detail ultrasonography for structural diseases.
Among the chromosome diseases, an average of 1/1000 frequently is the most common anomaly Down syndrome. It was first described by Landon Down in 1866. Its frequency increases with age. Down syndrome, which is included in the aneuploidy group, is characterized by the fact that one segment or all of the long arm of the 21st chromosome is 3. Most of the chromosome anomalies that are incompatible with life are lost in pregnancy and even in early pregnancy weeks. This is actually natural selection. Down syndrome is subjected to screening tests because of its important place among chromosome anomalies compatible with life.
Personalized Screening Criteria
The most important risk factor and screening test under this heading is maternal age. Although 70% of Down syndrome is in the female group under the age of 35, the most common screening test is still female age. Only 30% of children with Down syndrome can be detected using maternal age. If 35 years old threshold is accepted, 5-10% of the pregnant population will enter the risk group and invasive interventions such as amniocentesis will be recommended to this group. This threshold value greatly increases the number of invasive procedures; bringing the threshold to 37 years of age brought with it the discussion and meanwhile, it was possible to increase the threshold value by increasing the screening sensitivity of the blood screening tests, which we will talk about later.
Another personalized screening parameter is gestational age. As the gestational week increases, the anomaly rates decrease due to natural selection in the early weeks.
Previously giving birth to a baby with Down syndrome is a condition that increases the risk. It increases the age-related risk of the mother by 0.75%.
Screening Tests Used in Down syndrome identification:
1) 11-14 weeks screening (Dual screening test)
It is the most common and sensitive test that can be applied at the earliest period, performed for the purpose of Down syndrome screening.
It can be applied between 11 weeks 0 days and 13 weeks 6 days.
Personal risk factors are taken into consideration.
It includes three ultrasonography criteria including baby’s butt distance measurement, neck skin thickness measurement and nasal bone measurement. In addition, blood is taken from the mother candidate on the day of ultrasonography and papp-a and HCG values are determined. Risk factors that increase the probability of anomaly;
* maternal age is high
* neck skin thickness higher than 2.5 mm
* small nasal bone (nasal hypoplasia)
* papp-a value is below 0.4 mom
* HCG value is over 2.5 mom.
The 11-14 week screening test has a 90% detection rate with a 5% false positivity rate.
(false positivity: the risk of the test is detected even though it is a healthy baby.)
The detection rate of only papp-a and hcg values is 60-65%. Adding neck thickness increases the sensitivity to 77%. The addition of nasal bone hypoplasia to the increase in nuchal skin thickness increased the detection rate of the test from 77% to 90%.
When all values are gathered in a pool, double screening test; It will give a probability rate for 80% of age-related risk.
The threshold value is considered to be 1/300. Below 1/300 threshold value is evaluated as scanning positivity.
Increased neck skin thickness; It is not a special case for Down Sedrom. Because it can also increase in other chromosomal anomalies, Turner Syndrome, congenital heart diseases and diaphragmatic hernia.
Feature of ultrasonographic equipment requires training and experience. For this reason, some doctors may choose to have a triple scan.
2) Triple Screening Test:
It is no longer a test that is used outside of regions or institutions with insufficient ultrasonongraphy device quality. It is applied between the 15-21 weeks of pregnancy.
Personal risk factors are taken into consideration.
Again, ultrasonography is applied and the measurement called biparietal diameter is obtained for the test of the baby’s head.
HCG, AFP and AFP values are examined in the blood taken from the mother on the same day. An increase in HCG value, a decrease in uE3 and AFP values may occur in other trisomic fetuses as well as Down syndrome.
The threshold value is considered to be 1/270 and is considered as a scan positivity below 1/270. The false positivity rate will still be 5%. The detection rate of the test in these conditions is 57-67%.
3) Triple Scan Test:
In order to increase the sensitivity of the triple screening test, inhibin A was added to the biochemical markers in the blood taken and the sensitivity of the screening test was increased to 70-75% under the age of 35.
4) Integrated test:
It is based on the use of a 11-14 week screening test and a triple test and determining the risk screening probability with this combination. If pac-a and four scans are calculated together during 11-14 weeks of screening, it is referred to as an integrated test and the screening capacity can reach 85% of the age-related risk. If it is added to the nuchal skin thickness from 11-14 weeks, the detection rate will reach 90%.
Repeating the same test in the same pregnancy, in the same trimester does not increase test sensitivity. The application of the first and second trimester tests in the same pregnancy will increase sensitivity, but will increase the likelihood of amniocentesis by adding a 5% false positive rate in each test.