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06/10/2016

Testing, testing: Prenatal genetic screening

The June 2016 issue of Obstetrics and Gynecology includes a study of the conversations between patients and “Health Care Providers” about prenatal genetic screening (PGS). The objective of the study was to “assess how obstetric health care providers counsel patients regarding prenatal genetic screening and how these conversations influence patients’ screening decisions.” PGS refers to blood and ultrasound tests performed early in pregnancy to determine whether a fetus is at high risk for various chromosomal anomalies, especially the anomaly that leads to Down syndrome. Several findings of the study are troubling.

First troubling finding: providers’ counseling of patients about PGS lasts an average of 1.5 minutes.

Second troubling finding: False-positive rates of PGS are discussed so rarely that they could not be reliably analyzed in the study. The false-positive rate refers to how often the screening test is wrong when it suggests that an abnormality is present; when a test says the baby has Down syndrome, but the baby in fact does not have Down syndrome, it is a false-positive. The guidelines of the American College of Obstetricians and Gynecologists suggest that false-positive rates ought to be discussed in counseling about PGS.

But counseling about false-positive rates is itself troubling. The false-positive rate of PGS is somewhere around 5%. Most of us hear this and think, “OK, 95% of the time the test is right when it says that the baby has Down Syndrome.” However, to answer the question, “What percentage of the time is a positive test correct?” you can’t look at the false-positive rate; instead, you have to know something called the Positive Predictive Value. And for PGS, the positive predictive value is only 10%. That means that 90% of the time when PGS says “Down syndrome,” the baby does not have Down syndrome. (Numbers are higher for cell-free fetal DNA testing; however, this test is fraught with its own problems, and the study under consideration here did not include this test.)

Third troubling finding: In 51% of the discussions analyzed, health care providers did not clarify the fact that PGS is not diagnostic. PGS is a screening test, not a diagnostic test. The difference is crucial. Since 90% of abnormal PGS tests are wrong, an abnormal test requires a follow-up procedure to actually make a diagnosis. That follow-up test is usually an invasive test involving a needle into the uterus and a small chance of causing a miscarriage. I have known of several women who didn’t understand the screening nature of the test when they had it performed; when the test came back showing high-risk for a genetic abnormality, they then refused the more definitive diagnostic test. The result was either they spent their entire pregnancy in needless anxiety until the child was born without the anomaly, or they aborted the child on the basis of a false test result.

Why is all of this so troubling? Because medical tests are a big deal. Our understanding — or misunderstanding — of the results of medical tests shape how we see and understand ourselves. And PGS medical tests are a bigger deal than most; because if the results of this study are true, parents are frequently getting cursory explanations (1.5 minutes) from “providers” who give inadequate information about PGS tests. They are then using that (mis)information to make life-or-death decisions for the children they carry. If we’re going to practice eugenics, we should at least make sure that we’re doing so in a fully-informed fashion.

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