Test administration time
The triple test can be done between weeks 15 and 22 of pregnancy. The best results are obtained in the examinations performed between the 16th and 18th weeks.
Evaluation of the test
The levels obtained by the examination of the blood sample taken from the expectant mother are then processed with the help of a computer program. At this stage, variables such as smoking, weight and height of the expectant mother, which may directly affect the level of substances in the blood, are also taken into account.
The most important variable that can determine the level of the substances measured in the blood is the week of pregnancy at the date of the examination. For this reason, it is very important to know the gestational age, that is, the last menstrual period, correctly. When the last menstrual period is given incorrectly, for example, when the gestational age is included in the calculation, a value that is normal for the week the baby is in may be found to be lower or higher and may lead to errors in the interpretation of the test.
The obtained raw values are then entered into the computer program and the risk calculation is made.
NTD risk assessment
For neural tube defect (NTD) risk estimation, the AFP value alone is compared with the week of gestation, and the risk estimation is made after adjustments for the weight of the expectant mother. In cases of NTD, more than normal AFP hormone passes through the defect in the form of an opening in the structure that covers the neural tissue, into the amniotic fluid and then into the blood of the expectant mother. Therefore, the AFP value is higher than it should be according to the gestational week. The higher the height, the higher the risk of NTD in the baby. AFP is high in the blood of the expectant mother in cases where the baby has an opening in the anterior abdominal wall such as twin and multiple pregnancy, kidney diseases in the baby, omphalocele and gastroschisis. Most of these conditions are relatively easy to spot on ultrasound.
The most important point is that the gestational week is correctly determined and entered into the computer correctly at the time of the test. If the gestational week of the expectant mother is greater than expected, then the AFP will be high and the NTD risk will be higher than it actually is in the test. But if the expectant mother is not satisfied, nothing is said.
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 it should be according to the gestational week, and HCG is higher than it should be. The computer program determines the deviations of these three hormones from the normal according to the gestational week and weight, and determines the risk of carrying a baby with Down syndrome in the expectant mother with age.
Interpretation of the test
It is decided whether an advanced research is needed, depending on whether the risk ratio obtained as a result of the evaluation made by the computer program is within acceptable limits.
Advanced testing means amniocentesis. Amniocentesis is not a test that should be done in every pregnant woman. When deciding who can and should not do it, the decision is made by considering triple test, double test, ultrasound findings, family or medical history.
To explain with an example: Let’s assume that the risk of Down syndrome is reported as 1:2048 as a result of the test. This result does not tell us whether the baby has Down syndrome. In this report, it is said that only 1 out of 2048 women who have the same characteristics as the mother-to-be in the example gave birth to a baby with Down syndrome, so the probability of this mother giving birth to a baby with Down syndrome is 1 in 2048.
The accepted normal risk limit for Down syndrome is 1/280. 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, it should be observed that the probability of the baby being healthy is approximately 40 times higher than the probability of having Down syndrome.
The test is considered positive if the risk is higher than the accepted limit.
Reliability of the test
As with all scientific reviews and screening tests, the triple screen test has the potential to be wrong and false positive or false negative results can be obtained. If the test is positive even though the baby is normal, it is a false positive result; In other words, it is a situation where the risk of finding an anomaly in the baby is high. A false negative result is called a negative test result even though the baby has Down syndrome, trisomy 18 or NTD.
The triple screen test has a 60% chance of catching Down syndrome, with a 5% chance of false positives. 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 catching Down syndrome is about 50%. While the rate of catching is 80% in expectant mothers who are over 35 years of age on the expected delivery date, the false positive rate reaches up to 25%. However, these numbers are not absolute values and different results have been reported in different studies.
Who is the test applied to?
Although the risk of giving birth to a baby with Down syndrome increases proportionally with increasing age, this test should be applied to all pregnant women as it is known that mothers under the age of 35 also give birth to a baby with Down syndrome.
Application in multiple pregnancies
Some experts argue that this test may give a good result in twin pregnancies, but it will not give a result in pregnancies with more than two babies, while others argue that it is difficult to determine the exact number of hormone levels in all multiple pregnancies, including twins. They argue that this test will not give very meaningful results, and instead recommend fetal nuchal translucency measurement.
Triple test for women over 35 years old
There is a direct relationship between increasing maternal age and the risk of giving birth to a baby with Down syndrome, and the age at which this risk increases most significantly is 35 years.
As a result of changing living conditions, increasing urbanization rate and women’s participation in business life day by day, the number of pregnant women over the age of 35 is constantly increasing. Therefore, the application and reliability of such screening tests are gaining more and more importance day by day. In fact, some experts argue that amniocentesis should be applied to every pregnant woman over the age of 35.
Positive test result
The first of these is to perform amniocentesis and to determine definitively whether Down syndrome is present, regardless of maternal age and ultrasonographic findings. This is the most preferred application when positive test results are obtained.
The second way is to search for ultrasound findings that may be related to the conditions that caused the test to be positive, by performing a detailed ultrasound examination, and to decide on the application of amniocentesis in the light of these findings.