◆What is NIPT (new prenatal diagnosis)?
NIPT (New-Invasive Prenatal Diagnosis) is a test that analyzes blood samples from the mother during pregnancy to determine chromosomal abnormalities in the fetus, such as Down syndrome (21 trisomies), Edwards syndrome (18 trisomies), and Patou syndrome (13 trisomies). The test uses state-of-the-art technology to detect all chromosome counts and can also identify partial defects and duplications. called Non-Invasive Prenatal Genetic Testing (NIPT), this test is a non-invasive screening test for common chromosomal In addition to common chromosomal disorders, sex chromosome abnormalities and microdeletions can also be investigated. Sex chromosome testing may also reveal the sex of the baby.
NIPT is characterized by its extremely high sensitivity and specificity compared to conventional prenatal diagnosis, and the accuracy of the test itself is said to be nearly 100%. However, the test does not provide a definitive diagnosis of chromosomal abnormalities; it is a nonconclusive test that determines whether there is a high (positive) or low (negative) possibility of having a chromosomal disorder. Therefore, even if a positive result is obtained with NIPT, it is recommended to undergo a chorionic villus test or amniotic fluid test as a definitive test. False positive or false negative results are also possible in rare cases.
This test is performed by analyzing fragments of DNA derived from the placenta present in the pregnant woman's blood. Since the placenta is genetically identical to the fetus, this DNA can be examined to obtain genetic information about the fetus. Modern advances in high-speed genetic analysis technology have facilitated the analysis of these DNA fragments, and in NIPT, fragments of placental-derived DNA that have passed into the mother's blood are analyzed to determine if any chromosomal abnormalities are present.
This is an important option for many pregnant women because fragments of placental-derived DNA are suspended in the mother's blood during pregnancy, and analysis of these fragments can provide early information about the health of the fetus and certain genetic disorders.
◆Principles of NIPT (New Prenatal Diagnosis)
The principle of NIPT (New-Information Prenatal Diagnosis) is to use DNA fragments of fetal origin present in the maternal blood during pregnancy to test for the presence of chromosomal disorders. In this test, DNA of fetal origin, not of maternal origin, is the target of analysis. Specifically, cell free DNA (cfDNA) derived from the placenta that flows into the bloodstream is collected and analyzed in detail using a sequencer. This cfDNA is used to detect possible chromosomal disorders.
A sequencer is a device that reads DNA sequences, and the latest technology uses next-generation sequencers (NGS). This technology accurately reads the four base sequences of DNA: A (adenine), T (thymine), G (guanine), and C (cytosine). The bases are essentially present in pairs (A and T, G and C are paired), which form the double helix structure of the DNA. The sequencer reads the extracted DNA sequence one by one to identify genetic abnormalities.
Conventional sequencers can read only one DNA sequence at a time, which is time-consuming and labor intensive. NGS technology, however, creates a DNA library and can read millions to billions of nucleotide sequences at a time. This technology enables fast and accurate sequencing of even small quantities of DNA, making it possible to efficiently detect chromosomal aberrations even in minute amounts of DNA samples such as cfDNA derived from fetuses.
The MPS method (massively parallel sequencing) is the standard method of analysis for NIPT. This is an NGS-based analysis method that quantitatively analyzes the amount of DNA components derived from each chromosome using cfDNA derived from the fetus. For example, if a fetus has a 21 trisomy, the amount of DNA derived from chromosome 21 is 1.5 times higher than normal.
Ministry of Health, Labour and Welfare NIPT: noninvasive prenatal testing Noninvasive prenatal genetic testing citation
◆Comparison with conventional prenatal diagnosis
Compared to conventional prenatal diagnostic methods, NIPT is characterized by its extremely high sensitivity and specificity and superior accuracy. However, the positive predictive value of 18 and 13 trisomies tends to be lower than that of 21 trisomies. There is also a risk of false positives (when the test result is positive but there is no actual chromosomal abnormality). Therefore, it is recommended that a positive NIPT result be followed by a confirmatory test such as a chorionic villus test or an amniotic fluid test.
◆Detection of microchromosome aberrations
Microdeletion syndrome is a disorder caused by deletion of a gene at a specific chromosomal location. The syndrome results from very small chromosomal changes, which can exist from at least 500,000 bases and are usually caused by deletions of 2 to 3 million base sequences. Depending on the location and size of the chromosome deletion, the type and severity of symptoms may vary, but in most cases it can cause mental retardation and developmental delay.
Most microdeletion syndromes are spontaneous and are not significantly related to maternal age. This means that the condition is caused not only by genetic factors, but also by environmental factors and accidental chromosomal changes. For this reason, the syndrome can occur in pregnant women of any age, from young to old, and it is not appropriate to limit testing to pregnant women of a particular age group.
Detection of microchromosomal abnormalities, especially microdeletion syndromes, can be difficult in general diagnostic facilities, but our laboratory is able to detect these abnormalities using advanced genetic testing techniques. Therefore, testing at a medical facility with expertise is recommended for accurate diagnosis and management of microdeletion syndromes. Such testing can play an important role in identifying potential risks early and providing necessary medical intervention and support.
◆Advantages and disadvantages of NIPT
- No risk to the fetus
There is no direct risk to the fetus because the test is performed only by drawing blood.
- Early testing is possible
It allows for earlier testing compared to conventional testing methods.
- High inspection accuracy
The high sensitivity and specificity of the test results make it possible to obtain more accurate test results.
- Risk of false positives
Since this is a screening test, there is a possibility of false positives.