He triple screening, also called triple test it's a screening or tracking test (in English, screening) that is done to the pregnant woman to detect possible genetic alterations of the fetus.
It is a non-invasive test, because it does not penetrate the uterus, which is made from a blood sample from the mother. It is a routine intervention that consists of a simple blood draw without risk to the mother or the fetus.
It is not a conclusive test, that is, it is not a diagnosis, but rather measure a risk index that the fetus has certain chromosomal abnormalities such as trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome) and neural tube defects.
Triple screening in the first trimester of pregnancy
It consists of an assessment of the risk of chromosomopathy that is obtained by combining three biochemical markers present in the mother's blood (hence its triple screening name): PAPP-A (alpha-fetoprotein, protein produced by the fetus), free beta-HCG (human chorionic gonadotropin, the pregnancy hormone, produced by the placenta ), and free estriol (estrogen produced by the fetus and placenta).
In some laboratories a fourth parameter, inhibin A, is also measured, which is called a quadruple detection test, but it is not too widespread.
These biochemical values intersect with the data from the measurement of nuchal translucency (nuchal fold) of the fetus determined by ultrasound and are weighted according to demographic data (mother's age, race, weight, if she is a smoker, or has diabetes ) throwing a control algorithm that measures the chances that the fetus has an anomaly.
When the risk is greater than 1 in 270 (reads 1/270 or 1: 270), that is when the value is less than 270, or what is the same is below the cut, it is recommended to perform more precise tests such as an amniocentesis or a corial biopsy to rule out or confirm the risk. In the next post we will talk more in detail about the values of triple screening.
In women under 35, it detects between 75 and 85 percent of neural tube abnormalities and 60 percent of Down syndrome cases. In women over 35, it detects 75 percent of neural tube abnormalities and cases of Down syndrome.
In addition to being used to detect chromosomal abnormalities, triple screening serves to identify a multiple pregnancy and pregnancies that are more or less advanced than previously believed.
Who is tested for triple screening
The priority object of screening is women with low risk of being carriers of a fetus affected by chromosomopathy, however, it does not exclude those who have high risk factors such as:
- Be over 35 years old
- Having a history of previous pregnancy with chromosomal abnormality
- Having a history of Down syndrome in the family
- That one of the parents bears a chromosomal abnormality
- Having had repeated abortions, children born dead or with congenital malformations without established causes
- Having been exposed to high levels of ionizing radiation
When triple screening is done
The triple screening of the first trimester is performed between week 10 and 13 of gestation, being between week 11 and 12 when it yields a more reliable result, with a diagnostic sensitivity of between 86 and 90 percent and 5 percent of false positives.
Occasionally, the test is performed at the beginning of the second trimester, between weeks 15 and 18, (ideally between 15 and 16) and can be done until week 20, but the sensitivity is lower, round 78 percent with a 14 percent false positives. At this stage of pregnancy, the time for the practice of amniocentesis should also be considered, if applicable, and the legal limit for voluntary termination of pregnancy, if this decision is considered.
In this first post we wanted to give you complete information about what it is, what it detects and to whom the triple screening test is performed or triple test. In future posts we will talk more in detail about how to interpret the triple screening values and what happens when the result indicates a high risk of the baby suffering from any chromosomal abnormality.
Photos | virexmachina and Neeta Lind on Flickr
In Babies and more | Screening test to detect down syndrome: a double-edged sword, Prenatal tests: triple screening
