Genetic screening is a burgeoning field of medicine, and family physicians will be instrumental in translating this new science to patients. Dr. Newberger's article1 in this issue of American Family Physician reviews the importance of screening for Down syndrome. The article is timely and the issues of sensitivity, specificity and cost-effectiveness—common to all screening tests—are reviewed.
On the surface, triple analyte screening for Down syndrome seems like a good idea. It is a blood test that can lead to prenatal detection of Down syndrome. But, like all screening tests, it has its drawbacks. Triple analyte screening has limited sensitivity, with an overall rate of 60 percent and an overall specificity of 93 percent, although these percentages vary according to the age of the patient being screened. A positive test result may need to be confirmed with amniocentesis. Following amniocentesis, a small number of fetuses with false-positive triple-analyte test results will be miscarried.
The limitations of this test highlight the importance of informed consent. The more complex the screening test with respect to potential adverse outcomes, the greater the physician's responsibility to discuss the alternatives, risks and benefits, including assessing each patient's understanding and preferences.2 Some patients want to know whether their infant has Down syndrome in order to emotionally prepare for the birth of that child. Other patients may want the option of termination, while still others are not interested in screening if the confirmatory test is associated with any risk of fetal loss. Shared decision-making ensures that the patient is invested in the decision to screen, regardless of the outcome.
While the potential benefits of better patient-physician communication in this litigious society are great, the burden of informed consent rests heavily on physicians. Can we provide this discussion to all pregnant patients? What are the practical considerations of time and financial reimbursement? How do we weigh the costs and benefits of screening for Down syndrome?
In 1996, the American College of Obstetricians and Gynecologists (ACOG) recommended triple analyte screening for all pregnant women less than 35 years of age and amniocentesis for all pregnant women 35 years or older; the U.S. Preventive Services Task Force (USPSTF) recommends triple analyte screening for all pregnant women. The USPSTF recommends offering the triple screen and amniocentesis to women 35 years or older.3,4 Decision analysis incorporates all potential benefits and costs. It has shown that implementation of the ACOG guideline is associated with the highest number of pregnancy losses associated with amniocentesis and the highest cost per case detected. Screening all women first with the triple analyte test has a 20 percent lower cost and almost one half the number of pregnancy losses associated with amniocentesis.5 This analysis suggests that the USPSTF recommendation would be more cost effective.
In addition, the detection rate of the triple analyte test is strongly age-dependent. Older patients accrue the greatest benefits of screening. There are low benefits or even negative effects associated with screening in younger patients. Decision analysis suggests that limiting screening to older women (e.g., those more than 30 years of age) would limit the number of women who need pretest counseling and the number of amniocentesis-related fetal losses, and would lower screening program costs.6,7
In the future, ultrasonography may be used to reduce the positive screening rate by a “genetic sonogram.” Differences in technical accuracy limit the widespread use of this type of ultrasonography.
With the proliferation of new genetic tests that detect carriers of genes that cause and predispose to disease, genetic testing will likely become a much bigger part of primary care practice. The impact on our practices may be profound. Family physicians must play a leadership role in the creation of guidelines for genetic testing that take into account all costs and benefits and in the promotion of skills that enhance communication and shared decision-making with our patients.