What is the reason for failing Down syndrome screening?

Down syndrome screening is abbreviated as Down screening. It mainly checks whether the baby has the possibility of Down syndrome by testing the mother's serum. The accuracy of this test is about 70%. Some people will find that they fail the Down syndrome screening process, which will make people particularly worried and afraid that they have some serious problems. In fact, for this examination, the first thing is to look at the reason, and then look at the time and method of the examination, consult relevant doctors, etc.

Down syndrome screening classification and examination time:

Pregnancy 15-20

Mid-term Down syndrome screening is performed at 6 weeks. Mid-term Down syndrome screening can calculate the probability of a baby suffering from neural tube defects, trisomy 18 and trisomy 21. Since the NT value is not involved in the calculation, its evaluation efficiency for trisomy 21 is lower than the early Down syndrome screening, but its screening scope is more comprehensive.

Interpretation of high risk or critical risk in Down syndrome screening:

Down syndrome screening is a screening test for children with trisomy 18, trisomy 21 and neural tube defects. Among them, children with trisomy 18 are often difficult to survive, while children with neural tube defects can be further diagnosed through systematic ultrasound around 22 weeks. Only children with trisomy 21, on the one hand, have a high chance of survival. On the other hand, the surviving fetuses will be born with congenital idiocy, which will greatly increase the social and family burden after birth, and are the main target of Down syndrome screening.

The birth rate of children with trisomy 21 and Down syndrome in my country is about 1/1000, while the detection threshold for Down syndrome screening set internationally is 1/270. Calculated by relevant formulas, a disease risk higher than 1/270 is considered a high risk for Down syndrome screening; a risk between 1/270-1/1000 is considered a critical risk for Down syndrome screening.

The accuracy rate of Down syndrome screening is about 70%, that is, 70% of fetuses with trisomy 21 will have high-risk results in Down syndrome screening, while the remaining 30% of sick fetuses often show critical risk in Down syndrome screening (a very small number show low risk). Therefore, those who are at high risk or borderline risk in Down syndrome screening need to undergo further genetic screening.

Down syndrome screening high risk or borderline risk

There are many factors that affect the results of Down syndrome screening (as mentioned above, it is affected by the pregnant woman’s age, last menstrual period, weight, whether she smokes, whether she has any disease, etc.), and the false positive rate is as high as over 90%. Among fetuses with borderline risk for Down syndrome screening, only about 1/200 actually have trisomy 21; for those at high risk for Down syndrome screening, the true positive rate of diagnosis is only 1%-2%. Therefore, a high risk or borderline risk in Down syndrome screening alone is by no means a clear indicator that there is a problem with the baby.

If your baby is at high risk or borderline risk in the Down syndrome screening, expectant parents, please do not worry too much, as the probability that your baby really has a staining problem is not high. Go to the hospital for treatment in time and follow the doctor's advice to undergo the next step of non-invasive DNA or amniocentesis testing, so that you can know more clearly whether your baby has trisomy 21. The non-invasive diagnostic accuracy of trisomy 21 is over 99%, and amniocentesis is close to 100%.