Translucency false positives
“The study of nuchal translucency, is a national examination, is non invasive, but it still does not offer full assurance as amnio/ villocentesis...”
Contrary to the truth, there is a widely shared belief, which puts the first trimester screening at a lower and limited level compared to invasive procedures and despite the risk of the latter, it is still perceived as the most complete and definitive test.
The reliability of this prenatal test can best be understood if we define better the need for having the screening: a woman will want to know the definitive answer as early as possible.
Amniocintesis can only give a definitive answer (at 17-18 weeks, by the time full results are available) only for chromosomal defects, which has a much lower probability of occurrence compared to the congenital heart malformation for example.
DON’T BE SCARED OF FALSE POSITIVES! WHAT ARE THE “FALSE POSITIVES OF THE TEST?
In order to conduct a test, firstly it is necessary to select a group of patients who are in the high risk category for the disease in question. This group of people are said to be “false positive”. This is a statistical term, which is often misunderstood and that has created unfounded fears.
The term “false positive” refers not to an error in examination, but the number of patients that have tested positive to the test and that are referred for further testing. The term “false” is seen as an error in the test.
In order to understand this, consider the following: during seasonal sales, if I go to a shop and choose some hats to try on, in statistical terms, for the shop owner the hats represent “false positive to sell”. In other words, the fact that they are of interest to the buyer, does not indicate that they have been sold... but that out of all the hats I am trying on, rather than others which I have not tried, I may decide to buy one.
For example when we say that the criteria for being offered amniocentesis is being above 35 years of age, we have concluded that all women above this age are of high risk, and therefore using statistical terms, all of this women are “false positive” as only very few of them will test positive for pregnancy with chromosomal defects.
Given that in Italy approximately 20% of pregnant women will have a child after the age of 35, the criteria for being offered amniocentesis can be defined as having a “false positive” value of 20%.
We should always limit the number of “false positive” without reducing the test’s precision. This way it is possible to minimize unnecessary worries to the pregnant women and possible complications from invasive procedures.
If maternal age, nuchal translucency measurment, nasal bone and blood tests are used as criteria for determining whether a patient should be referred for amniocentesis, the false positive rate (patients that will be referred for further testing), is only 2.5% (against the 20% in Italy) and similarly, it is possible to identify 95% of fetuses with Down syndrome against the 30% when only maternal age is used as criterion for further investigation.