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MS WordPerinatologist Corner - C.E.U/C.M.E. Modules
Prenatal Genetic Screening – Serum and Ultrasound
Sponsored by The Indian Health Service Clinical Support Center
3. Screening versus Diagnosis
Case Scenario
BT is a 25 y/o G3P1 at 17 weeks gestation. Her multiple marker screen returns positive for fetal Down syndrome with a risk of 1 in 74. What constitutes appropriate management and counseling at this point?
Screening versus Diagnosis
Perhaps the most important fact to emphasize when counseling patients is that this is a “screening” test not a “diagnostic” test. Patients are used to “all or nothing”, “pass-fail” tests. They go to the Department of Motor Vehicles and either obtain a driver’s license or not depending on whether they pass or fail their test.
It is very important that the patient understand that triple testing is a “screening” test, not the type of “all-or-nothing” test that determines whether her baby is normal or not. It is sometimes helpful to continue the analogies and explain that it this type of testing is like going fishing. For instance, you may want to catch herring. You go to a likely spot and cast your net. It comes back with some herring, but also a lot of other kinds of fish you didn’t want. It also doesn’t catch all the herring in the sea!
A screening test
is helpful to identify people with an increased risk of a certain condition, but the majority of people do not actually have the condition. Likewise, this type of test is not able to identify everyone who has the condition. Understanding this, does she want to have the test and the information it may provide? The time spent making this distinction will provide an important foundation should you have to inform the patient a few days later that she has an abnormal result.
Amniocentesis is the definitive diagnostic test
and has an accuracy of 99.5% for the detection of abnormal fetal karyotype, or chromosome makeup. As an invasive procedure, it has an associated risk of about 0.5% (1 in 200, or, “199 chances out of 200 that everything will be fine!”) for procedure-related pregnancy loss. It is not a “rite of passage” that all pregnant women over age 35 necessarily have to have if they don’t so choose. Approximately 40 amniocenteses are necessary to detect one case of DS. That is partly because, depending on the test and the patient’s age, 3-20% of all MMS return positive. This is the “fixed false positive rate” applied so as to enhance the sensitivity of the screening. Not all of these results may merit amniocentesis. MMS looks for fetal neural tube defects (NTD), and for fetal Down syndrome (DS). Let’s discuss each of these entities separately and look at “the in’s and out’s” of the various testing schemes.
