How Screening Procedures Differ From Diagnostic Testing

A ProPublica article reports that pregnancy screening tests are unregulated and refers to these screening tests as the “Wild Wild West” of assessment procedures.

In this article, ProPublica reporters tell the story of Amanda, a 32 year-old woman who delivered a baby with trisomy 13, an extra chromosome (3 instead of 2) on the 13th chromosome. This condition is not compatible with life. Amanda had had a “screen” blood draw commonly done on pregnant women. About 40 percent of pregnant women get this screen.

It appears to me that the authors of the ProPublica article do not seem to understand, and the patient evidently did not understand, and perhaps some providers do not understand the difference between screenings and diagnostic tests. Providers need to successfully relay the difference between a screening test and a diagnostic test to their patients. Had Amanda understood the difference, she might have had a better understanding of why her screening test did not show up her baby’s trisomy 13.  The reporters, at least, should have understood the difference.

The important definitions central to this article are screening sensitivity and screening specificity. An understanding of these two differences are critical in a situation such as Amanda’s. Sensitivity means the ability to find the condition that the screening is looking for. By definition, screening is very sensitive and therefore you find, in a large group of people, many conditions besides the one you’re looking for. The classic PAP smear is a good example. The PAP smear looks for cervical cancer, but also picks up vaginal cancer and even infections such as bacterial vaginosis. The PAP test can report a positive result even though the positive result is not cervical cancer. So, the PAP test will return a large group of true positives for cervical cancer, but also a small group of false positives for those who don’t have cervical cancer but have perhaps bacterial vaginosis. This scenario is central to the issue of what screening tests mean.

Specificity means finding the conditions you’re looking for and only the condition you are looking for. The diagnostic test provides this kind of specificity. In other words, the diagnostic test is more accurate for searching for a specific condition. The diagnostic test is more expensive and puts the patient at greater risk because diagnostic tests are more invasive. If a PAP smear comes back positive, a diagnostic test needs to be done to determine the precise condition. To separate out the wheat from the chaff with a PAP test, the next step would be a colposcopic exam with a diagnostic biopsy. This procedure is more expensive and adds more risk, but it does decrease the false positive rate so that the diagnostic test verifies the accuracy of the PAP screen for the presence of cancer.

Today obstetrical screens have become more accurate, and have become more common. These screens are less risky because they involve only a blood draw, not a surgical procedure.  The ProPublica article indicates near its conclusion that it is the doctor’s or the midwife’s task or obligation to explain to the patient the meaning of a screen test versus the meaning of a diagnostic test; that is, the difference between sensitivity and specificity. I agree. Who else could inform the patient of this difference? Certainly, not the lab.

Currently, the Federal Drug Administration is concerned about investigating the false positive results in screening tests. The problem with Amanda’s test is that it is a false negative. The patient, Amanda, should have been told that this was a screen test and not a diagnostic procedure. So, there is bound to be less accuracy. If there was an understanding of this difference, the ProPublia reporters would have had no reason for this article. It’s not the results of the screening test that are the problem. It’s that Amanda did not receive the information she needed from her provider about the difference between a screening test and a diagnostic test, and that her test could be a false negative.

To be crystal clear, this screen was a false negative, indicating there was no disease when there indeed was disease.

From the prenatal laboratory assessment screening perspective, the entire area of obstetrics has moved to screening tests with less risk, but at the same time, less accuracy in the results. I’m not certain how often that idea is actually explained to the patient because it is a complicated notion and it is time consuming to explain. I take the time in my recent book, Pregnancy Your Way: Choose a Safe and Happy Birth, to explain the difference between the prenatal screening test and the prenatal diagnostic test. Prenatal patients need to understand that the only time they can get a diagnosis is if the more invasive and riskier diagnostic test is performed. Amniocentesis or chorionic villus sampling (CVS) are diagnostic tests and can on rare occasions trigger a miscarriage, something a blood draw for a screening test won’t trigger.

Granted, for many women, the screening test adds some kind of peace of mind, but only if it’s accurate, which it will inevitably be most of the time. The screen is offered more often to patients today on the basis of increasing safety, the increasing accuracy of current screens, and perhaps sometimes because of the lower expense.

Forty years ago, when screens first started appearing on the obstetric scene, I recommended against screening to my patients precisely because of the possible false negatives, as well as the possible false positives. I had no cause to regret my decision about recommending against screening for my patients unless family history indicated otherwise.