Pregnancy Screening

December 14, 2007

Q: The more I read about the triple screen and other prenatal tests, the angrier I get! My husband and I went through those ridiculous tests, which came come back saying we could have a Down's Syndrome baby. Then we went through the amniocentesis--and all of the crying and stressing and the hoping that our baby would be okay. And magically the results did come back okay. It's a scam--another way for doctors to make money! Anyway, can you do an article on some of those useless tests? I think, like us, new parents-to-be have no idea what the statistics are on some of these tests having such inaccurate results a majority of the time.

A: The test you describe is a simple blood test which can be evaluated for something called maternal serum alphafetoprotein (MSAFP). MSFAP is a protein made by the fetal yolk sac in early pregnancy and later on by the fetal liver. It can be measured in the Mom-to-be's blood and studies have shown that certain levels can indicate what pregnant women may be at risk for neural tube defects (like Spina Bifida), Down's syndrome, and other birth defects. Approximately 80-90% of certain defects of the baby's spine can be caught by the routine screen of the MSAFP. Unfortunately, we doctors do a very poor job in informing our patients about the difference between a "test" and a "screen." A patient is likely to walk away from an appointment with orders for both.

 

What Are The Pregnancy Tests Used?

A "test" is a diagnostic tool used to do just that - make a diagnosis; whereas a "screen" is nothing more than a method to inexpensively yank out of the general population people who need to be evaluated more closely. The word screen is more than symbolic. Think of a patio screen that lets air and moisture through but not insects. A medical screen, similarly, selectively collects those patients to be identified for certain problems. But just as some harmless insects can be stopped at the porch while smaller pests can still get in, so also, depending on the selection criteria of a medical screen, some people are falsely identified as being at risk while some may be missed altogether. In other words, a screen is a pretty crummy test, but it's value is in its ability to narrow down the more expensive testing to only those at an increased risk. The same objections you raise about the MSAFP can be just as easily raised for Pap smears, mammograms, TB screening, and blood tests for prostate cancer. As far as the MSAFP goes, if everyone is screened there will be about 5% abnormal readings; further testing¡VREAL testing¡Vwill then go on to show that only 3-5% of these (3/1000 babies) will have a real problem. In Down's Syndrome screening, only 20-30% of affected babies will be caught with this screen. Besides these inherent inaccuracies, there's also a dependency on arithmetic. The pregnancy has to be dated accurately, with the MSAFP being performed between 15 and 20 weeks gestation, generally. If the dates are incorrectly calculated, then the MSAFP may have a further inaccuracy added to the mix. Herein lies the crux of your complaint.

Yes, the MSAFP screen or any screen, for that matter--is inaccurate. Yes, it can miss a problem, and yes, it can also give false reassurances. But for these purposes the MSAFP is the best screen we have so far. The alternative is to do the more expensive but exact testing on everyone or nothing at all. But the former would put a lot of babies through the unnecessary risks of an unnecessary amniocenteses, and the costs of prenatal care would make insurance premiums ridiculous; and the latter doing no screens would give us no information.

In my practice every pregnant woman is offered the test, because it is considered the standard of care. But of course it is optional. Patients with a strong pro-life view, for instance, often decline, stating that they wouldn't opt to terminate a pregnancy anyway. Others who feel the same way may nevertheless want to prepare themselves and so opt for the test. Still others feel the need to know if everything is alright. When patients ask me what I recommend, I have to tell them that it is as much a philosophical decision as a medical one. The parents-to-be must decide. Any abnormalities are then referred to a perinatologist (specialist in Maternal-Fetal medicine). Pregnancy is a gamble, but most don't know this until the dice are actually thrown. The uncertainty inherent in a screen like the MSAFP doesn't help the "crying, stressing, and hoping" that accompany this uncertainty. Although the screens and tests we have today have come a long way, still we know there's a long way to go. In today's world of managed care and cost containment, I'm afraid the screen is here to stay. But a patient's doctor can go a long way to allay anxiety by making it clear how it differs from an actual diagnostic test. And I would hope that a patient would be delighted that the baby is normal no matter how much crying and stressing the screen caused.

 

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