Double test
In medicine, the double test is an examination method of prenatal diagnostics in which the measurement of two (double = double) laboratory values in the mother's blood can be used to calculate the probability of physical and chromosomal malformations in the unborn child, in particular for trisomy 21 . A distinction is made between a "double test" in the first and second trimester.
Time of examination
The double test in the first trimester can be carried out between the 10th and 14th week of pregnancy . In the second trimester, this phase is usually indicated with the 14th to the full 20th week of pregnancy. It is not a routine examination and is only carried out if the pregnant woman or the parents expressly request it.
What is being investigated?
The examination during the first trimester includes an analysis of two maternal blood levels , a specific pregnancy enzyme (PAPP-A: Pregnancy associated plasma protein A , English for pregnancy-associated protein A ) and the free β-subunit of the pregnancy hormone human chorionic gonadotropin (free β- hCG ). The examination in the second trimester includes an analysis of the two maternal blood values hCG and AFP .
PAPP-A, hCG and free β-hCG are products of the placenta ( placenta ). In contrast, AFP is first formed in the fetal yolk sac and later in the fetal liver .
Importance of blood values
A low PAPP-A value is associated with various fetal chromosome changes, in particular with trisomy 21. In Edwards syndrome (trisomy 18) or triploidy (tripling of all chromosomes), the PAPP-A values are usually extremely low, often even at the detection limit. The hCG or free β-hCG is often doubled in fetal trisomy 21 , but extremely low in almost all cases in fetal trisomy 18 . The AFP is usually reduced in a fetal trisomy 21, in the presence of a fetal closure defect , e.g. B. neural tube defect or abdominal wall defect , but significantly increased.
In addition, the maternal age, maternal weight, the exact week of pregnancy, any existing diabetes mellitus or gestational diabetes of the mother, her nicotine consumption (smoking), previous pregnancies with a child with chromosomal peculiarities and those during NT screening (measurement of the nuchal transparency ) values measured by ultrasound are used to provide a probability statement for peculiarities in the unborn child.
If these examinations (double test in the first trimester and neck transparency measurement ) are carried out in combination, they are summarized under the term first trimester screening or first trimester screening . Blood values (and, if necessary, the thickness of the baby's neck fold ) are evaluated in order to provide an individual probability statement based on statistical values for, in particular, Down syndrome (trisomy 21) in the baby.
criticism
The test is suitable for identifying those pregnant women who have a statistically higher probability of expecting a child with a particular chromosomal peculiarity, so that further diagnostics can be recommended to them. Since the double test is only a search test and not a diagnostic examination, this means that some of the children with a corresponding peculiarity are not recognized (residual risk due to a false negative result) and the test often gives false prognoses ( false positive result). Certain chromosomal peculiarities can only be diagnosed with a high degree of certainty, but also with certain risks (including that of a miscarriage ), through a chromosome examination (e.g. by means of a chorionic villus sampling or amniocentesis ) .
