The slaughter of the innocents

This image shows a possible result of the tests
described in the post. Black figures represent
pregnant women who tested negative.
Red figures are false positives.
Only the white figures are true positives.
Above-left are the results
for the Down syndrome

A study performed a few years ago [1] describes a delicate situation regarding certain widely used medical tests. The triple test has been performed on many pregnant women in order to detect whether the fetus will have the Down syndrome and other deficiencies. When the test is positive, gynecologists often recommend an amniocentesis or a chorionic villus sampling (biopsy), but as these tests involve some risk, many pregnant women do not want to do it, and some may decide to have an abortion based only on the results of the triple test.

The Down syndrome affects approximately 0.13% of fetuses. The problem is that the triple test used to detect it has a 70% sensitivity (or what is the same, the probability of a false negative is 30%) and a 91% specificity, which means that the probability of a false positive is 9%.

How should we interpret these numbers?

Suppose the triple test is performed on 1000 pregnant women. By applying the above data we conclude that:

•         The likely number of fetuses affected by Down syndrome will be 1 or 2.

•         The probability of detecting them with the test is 70%.

•         90 women (9% in 1000) will test positive, even if their child won’t be affected by the Down syndrome.

When the test is positive, the probability that the fetus is actually affected by the syndrome is 1 or 2 in 90: just over 2% (yes, so low!). If, as a result of this information, an abortion is performed, 98% of the time a normal child will be killed. If it had been allowed to be born, it wouldn’t have suffered the syndrome.

Things may improve thanks to a new blood test for pregnant women, lately implemented to detect fetal trisomies such as those in chromosome 21 (which produces the Down syndrome) with a sensitivity of 99% and a specificity of 99.9%. At least, thanks to this, some fetuses will escape abortion, but a fundamental question will remain standing: children with the Down syndrome have no right to life?

[1] E.Kurz-Milcke, G.Gigerenzer, L.Martignon, Transparency in risk communication: graphical and analog tools, en Strategies for risk communication: evolution, evidence, experience, Annals of the New York Academy of Sciences, vol.1128, 2008 

The same post in Spanish

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Manuel Alfonseca