This week, I would like to talk about the triple screening test, one of the screening tests we need to do to detect genetic diseases of the baby growing in the uterus after pregnancy detection.
The triple screening test is a blood test performed between the 16th and 18th weeks of pregnancy. In fact, although it can be done between 15-20 weeks of pregnancy, the best results are seen between 16-18 weeks of pregnancy. It is taken within weeks. In this test, it is called the triple screening test because 3 specific substances secreted from the placenta (the baby's partner) and the baby are examined; AFP, hCG, and Estriol
AFP: It is a protein produced by the baby.
hCG: It is a hormone produced by the placenta.
Estriol: It is produced by both the baby and the placenta.
Screening Test What is it?
Before explaining the triple screening test, I would like to explain what the screening test means. The screening test is not a definitive diagnostic test; It only helps detect risky babies. According to the results of the blood test, it gives the risk of the baby having a genetic disease. It is a test with false positives and negatives, meaning that a risky result does not necessarily indicate that the baby has a genetic disease or that a normal result does not indicate that the baby is 100% healthy. When risky results are detected, definitive diagnostic tests such as amniocentesis are required. The triple screening test's probability of detecting Down syndrome is approximately 60%, with a false positive probability of around 5%. These rates are directly related to maternal age. While the false positive rate is 4% in women under the age of 35, the rate of detecting Down syndrome is approximately 50%. For expectant mothers who are over 35 years of age on the expected date of birth, the detection rate is 80%, while the false positive rate reaches up to 25%.
Evaluation of the test
The levels obtained by examining the blood sample taken from the expectant mother are then recorded on a computer. processed with the help of the program. At this stage, variables such as the expectant mother's smoking, weight and height, which can directly affect the level of substances in the blood, are also taken into account.
The most important variable that can determine the level of substances measured in the blood is the gestational week at the time of the examination. For this reason, the actual gestational age must be calculated by verifying the last menstrual period and ultrasound.
When evaluating the test results Other issues taken into consideration are the age of the expectant mother, which can directly affect the risk of diseases, and whether there is a previous history of birth with anomalies.
The raw values obtained are then entered into the computer program to calculate the risk.
Who Should Have a Triple Screening Test?
Although it is required for all pregnancies, especially
• Those with a family history of children with disabilities or genetic diseases
• Expectant mothers aged 35 and over
• Those with a history of drug use that could potentially harm the pregnancy
• A triple screening test should be performed for expectant mothers who have diabetes and use insulin
• If they have had a virus infection during pregnancy. • If there is exposure to high doses of radiation, a triple screening test should be performed. It is seen in babies with brain and spine diseases, which we call neural tube defects, and some digestive and kidney diseases.
Low AFP and abnormal hCG and estriol levels occur in trisomy 21 (Down syndrome), trisomy 18 (Edwards Syndrome) and some chromosomal diseases.
NTD risk assessment
For neural tube defect (NTD) risk prediction The AFP value alone is compared with the week of pregnancy, and the risk estimate is made after corrections are made according to the weight of the expectant mother. In cases of NTD, more AFP hormone than normal passes through the defect in the structure covering the neural tissue into the amniotic fluid and from there into the blood of the expectant mother. In cases where there is an opening in the front abdominal wall of the baby, such as twin and multiple pregnancies, kidney diseases in the baby, omphalocele and gastroschisis, AFP is also high in the blood of the expectant mother. interest. Nowadays, these diseases I mentioned are detected with detailed ultrasound.
Down syndrome risk assessment
Down syndrome (Trisomy 21) risk estimation is made using the expectant mother's age, weight and three hormone measurements. In Down syndrome, AFP and uE3 are lower than they should be according to the gestational age, and HCG is higher than they should be. The computer program determines the deviations of these three hormones from normal according to gestational age and weight, and determines the risk of the expectant mother carrying a baby with Down syndrome with age.
Interpretation of the test
Computer pro Depending on whether the risk ratio obtained as a result of the evaluation made by the gram is within acceptable limits, it is decided whether an advanced level investigation is required.
What is meant by advanced test is amniocentesis. Amniocentesis is not a test that should be performed on every pregnant woman. When deciding who will have it done and who will not have it done, the decision is made by taking into account the mother's age, triple test, double test, ultrasound findings, family or medical history.
Detecting a high risk as a result of the triple test does not necessarily indicate that there is a problem in the baby, and the risk is low or even very low. It does not guarantee that the baby is healthy.
To explain with an example: Let's assume that the risk of Down syndrome is reported as 1:2987 as a result of the test. This result does not tell us whether the baby has Down syndrome or not. In this report, it is said that only 1 of 2987 women with the same characteristics as the expectant mother in the example gave birth to a baby with Down syndrome, therefore the probability of this mother giving birth to a baby with Down syndrome is 1 in 2987.
The accepted normal risk limit for Down syndrome is 1/280. is. In cases where the risk is higher (for example, 1/100 or 1/40), amniocentesis, which is an advanced examination, is recommended. Even if the risk is determined as 1/40, the probability of the baby being healthy is approximately 40 times higher than the probability of having Down syndrome.
If the risk is higher than the accepted limit, the test is considered to have given a positive result.
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Should a Triple Screening Test be performed in every pregnancy?
Although it is recommended in all pregnancies, sometimes the mother and father The candidate may not accept this test for various reasons.
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