Alpha-fetoprotein Test (AFP)
What is the AFP test?
The AFP test is a blood serum test that screens for a few different things:
Neural Tube Defects (Such as Spina Bifida)
How is the AFP test done?
The AFP test is done by drawing blood from the pregnant woman, either in the lab, the doctor’s, or the midwife’s office.
What do the results mean?
Most AFP tests will come back within normal range. If it comes back in a higher range, it may mean that your baby is at risk for some of the conditions listed above. However, this is only a risk-assessment, and further testing will be suggested. The AFP test can be valuable in catching a baby with Spina Bifida, so that the parents can take steps to plan for a possible cesarean delivery, as well as possible surgery for the baby.
However, it may be worth weighing the benefits and the risks of this test. The AFP test is known for a high rate of false positives, meaning that you are alerted by your doctor that your baby could have one of the conditions listed above and have more invasive tests ordered, when the baby does not have anything wrong at all. Most research shows an 80 /-% false positive rate. Some factors that may cause a false positive are a miscalculation of gestational age, or, maternal age, weight and the presence of diabetes. The gestational age is very important, because the amount of hormone in your blood will vary with gestational age.
Of the positive test results, 90% of these babies will not have any anomalies.
False positives may not seem like a problem to most, however, there are follow up tests that are done that do carry risks, like amniocentesis or chorionic villus sampling (CVS), which does carry a 1 – 2% rate of miscarriage. The standard protocol is usually to repeat the AFP or triple screen first. If another positive screen comes up, protocol will usually then go to the Level II Ultrasound. Ultrasound can even be used to detect some of the anomalies associated with Downs Syndrome. If the ultrasound doesn’t find a mix up in gestational age, or a multiple pregnancy, an amniocentesis is generally the next step.
Remember, after carefully researching each of the prenatal tests, you can absolutely agree to, or refuse, any prenatal test. It is important to weigh not only the immediate benefits vs. risks, but the possibility of more testing that may be ordered for followup as well.
Hyperglycosylated human chorionic gonadotropin (invasive trophoblast antigen) immunoassay: A new basis for gestational Down syndrome screening. Clin Chem. 1999 Dec;45(12):2109-19.
Integrated serum screening for Down syndrome in primary obstetric practice. Prenat Diagn. 2005 Dec;25(12):1162-7.
New England Journal of Medicine 323, No. 9, Aug 30, 1990.
Reproducibility of risk figures in 2nd-trimester maternal serum screening for down syndrome: comparison of 2 laboratories. Clin Chem. 2006 Nov;52(11):2087-94. Epub 2006 Sep 21. Benn PA, Makowski GS, Egan JF, Wright D.