DNA-based non-invasive prenatal screening
Non-invasive DNA-based prenatal screening, otherwise known as non-invasive prenatal test (NIPT), is a type of investigation that is based on the analysis of circulating fetal DNA present in maternal blood. For its execution, a simple maternal blood sample is required, which can be carried out starting from the tenth week of gestation. Its "non-invasiveness" has made this analysis very widespread. The risks are in fact zero for both the unborn child and the mother.
It is a "screening" test, which unlike a "diagnostic" test such as amniocentesis or other so-called invasive investigation techniques, allows you to formulate a prediction of low risk or high risk. The sensitivity (true positives) and specificity (true negatives), relative to aneuploidies such as Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18) and Patau syndrome (Trisomy 13) are very high. The validation studies of the test have in fact highlighted a specificity greater than 99% for Trisomies 13, 18, 21 and a sensitivity that is between 92 and 99%.
The guidelines of the Ministry of Health and the Superior Health Council in fact state that "the survey is currently targeted and validated for the main autosomal aneuploidies (T21, T18, T13)". In light of the foregoing, it appears important to emphasize that non-invasive prenatal analysis has very high potential for screening the 3 main aneuploidies of chromosomes; syndromes that alone represent 50-70% of the chromosomal aberrations possibly present in the fetus. It is therefore an excellent tool for prenatal screening available to the couple who must receive information to understand all the characteristics and limitations of the test itself.
Extended or second level non-invasive prenatal testing
Non-invasive prenatal diagnostic tests are currently available on the market to detect additional types of mutations, such as microdeletion / microduplication syndromes or other syndromes associated with structural chromosomal rearrangements. This type of test boasts levels of detail that in theory may seem much more accurate than the test that allows you to investigate the 3 main chromosomal aneuploidies (T21, T18, T13).
The guidelines formulated by the Ministry of Health and the Superior Health Council take into account the fact that the diagnostic efficiency for the three syndromes listed above is very high, justifying its validation and therefore its use in prenatal screening. On the contrary, for the microdeletion / microduplication syndromes or those associated with structural rearrangements the same Ministry states that “The preliminary results however indicate a low sensitivity (62-95%) […] and a high FPR. […] The validation process has shown some criticalities. In particular, most of the cases used for validation did not concern prenatal screening (plasma from pregnant women), but samples created in the laboratory, which simulated a pathology (PlasmArtTM). For this reason, the declared sensitivity and specificity are not representative of the actual performance of the test in the clinical setting. For example, considering the sensitivity and specificity values of the more optimistic test [...], the positive predictive value of the test (ie the probability that the microdeletion identified by the protocol considered was true) did not exceed 7%. " The study therefore concludes that “At present, NIPT-based screening has no reason to be extended beyond T21, T18, T13”.
What stands out is therefore the importance of a conscious and informed decision to accompany the woman and the couple on the path of pregnancy. A path that in itself contains, at times, states of uncertainty and concern that in no case should turn into a speculative tool. For those wishing to learn more about the subject, it is possible to consult the full text issued by the Ministry of Health at http://www.salute.gov.it/imgs/C_17_pubblicazioni_2381_allegato.pdf.)
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