Getting a False Positive on a Screening TestKristen J. Gough
During pregnancy, you’re poked and prodded to provide blood, urine, and ultrasound images, all in hopes of finding definitive answers about your body and your baby. Yet many tests you’ll experience during pregnancy aren’t yes or no tests—they can’t necessarily determine that you’ll have a healthy baby or that something isn’t right. Instead, many prenatal tests are looking for whether your baby is at a greater risk of having certain problems based on the results. Learning more about prenatal testing can relieve anxiety, especially if you are one of the many women who get a false positive.
Understanding Tests—Diagnostic vs. Screening
“The most important preliminary point to understand with all prenatal tests is the difference between a screening test and a diagnostic test,” explains Erica Lyon, author of The Big Book of Birth and the founder and director of Realbirth, a childbirth education center in New York City. “A screening test shows perhaps a possible increased chance of something existing whereas a diagnostic test determines if something does, in fact, exist.”
The Quad Screen
The quad screen is perhaps one of the most misunderstood of the prenatal tests. The triple screen (also known by several other names including the multiple marker screening and maternal serum screening) is a blood test administered between 15 to 20 weeks gestational age. It looks at the levels of four pregnancy hormones—hCG (human chorionic gonadotrophin), estriol, inhibin-A, and AFP (alpha-fetoprotein)—in your bloodstream; hormone levels outside of the normal range may indicate a problem.
“The [quad] screen is a screening test,” says Lyon. “The most important thing to understand specifically about the screen is that it has an inordinately high false-positive rate, meaning it often shows an increased chance, when in fact, absolutely everything is all right.”
Of those who take the test, five to seven percent will come back with a positive result, explains Dr. Mary E. Norton, director of the Prenatal Diagnostic Center at the University of California, San Francisco Medical Center. The quad screen assesses of the level of risk for having a child with Down syndrome, another chromosomal problem, or spina bifida and other neural tube defects.
“The thought process is that while [the quad screen] may not be very accurate, its primary value is that in a non-invasive way, since its only a blood test, it is somewhat helpful in screening for the rare case of a young woman who wouldn’t necessarily get more diagnostic testing, for example an amnio,” explains Lyon.
A New Screening Test
Another test gaining popularity is the first-trimester screening, which combines an ultrasound that measures the amount of thickening at the back of the neck (a sign of Down syndrome) with a blood test. The test is also known as the Nuchal Fold test. Since this test is relatively new in the United States, it may not be available in all areas, and those administering the test must be highly trained to provide the most accurate results. Yet, as a screening test you can still anticipate false positives.
Diagnostic Tests: Amniocentesis and Chorionic Villus Sampling (CVS)
A positive screening test result may indicate that you should consider additional testing to determine your baby’s health. A “diagnostic” test can make the actual diagnosis.
The amniocentesis and CVS tests involve more invasive procedures. For an amniocentesis, doctors insert a needle into the abdomen and uterus to extract amniotic fluid. A CVS involves placing a thin tube into the uterus through the cervix or through inserting a needle into abdominal wall. Then, a doctor suctions out a tiny piece of the placenta.
There are important differences with the two tests. An amnio can be performed at 15 to 20 weeks, and the results can take up to a month to receive. Alternately, a CVS can be done much sooner, at 10 to 12 weeks, and the results are available one to two weeks after the test. The advantage with both tests is that as diagnostic tests, the results are definitive, says Dr. Norton.
Neither test is without risks, most importantly miscarriage, although doctors point as these tests have become more routine over the years, the rate of miscarriage has lessened. Dr. Norton recently published results of incidences of miscarriage using CVS and amniocentesis. The study, which appeared in the September 2006 issue of Obstetrics and Gynecology, found “the risk of both CVS and amnio seems to be lower than the one in 200 risk that is commonly quoted.”
“I would suspect that the majority of pregnant women don’t really understand these tests [quad serum],” says Dr. Norton. “I think many believe that a ‘normal’ result means that their fetus is fine, and an ‘abnormal’ result means it most likely has a problem, when in fact, the test simply refines risk. Most fetuses with an abnormal [positive] result ultimately turn out to be fine, while some fetuses with a normal [negative] result turn out to have Down syndrome, or some other problem that the test doesn’t test for.”
Why Screening Tests Are Important
While the quad serum and other prenatal screening tests may make your life more stressful, these tests do give you and your healthcare provider important information about how your baby is developing.
Dr. Timothy R.B. Johnson, the chair of the department of Obstetrics and Gynecology at the University of Michigan Health System Hospital in Ann Arbor, Michigan, explains that the quad screen does more than just assess a baby’s risk factors for certain disorders. “Abnormal hormone levels are also associated with complications such as preterm labor.” So, a healthcare provider might manage your pregnancy differently based on the results.
Also an expert in maternal-fetal health, Dr. Johnson suggests that if results from an amnio or CVS indicate that your baby has a disorder, your doctor can put you in touch with support groups. Your healthcare provider may also choose to have you deliver your baby at a hospital that specializes in caring for infants with certain disorders.
While screening tests can provide more information about your growing baby, these tests are optional and many women refuse them. The thinking goes that if you get a positive on your quad screen, the next step is either to do nothing or to get a diagnostic test. Based on the results of the diagnostic tests, you either do nothing or choose to terminate the pregnancy. Since many women want to avoid the risk of miscarriage that comes with diagnostic tests and do not consider terminating the pregnancy an option, they don’t feel compelled do the screening.
Working with Your Healthcare Provider
As with any aspect in your pregnancy, it’s vital that you have a good working relationship with your healthcare provider. Dr. Johnson recommends that you listen carefully to how your doctor explains the results of your test. Dr. Norton adds that with most physicians seeing 40 patients a day, your healthcare provider may not have time to go into details about the implications of your test.
Dr. Johnson reiterates that you shouldn’t hesitate to ask your healthcare provider questions—and make sure you understand the answers. Ask for written information, reliable websites, and the proper phone number in case you have any further questions.