A Serum screening test (triple or quad screening test) is a blood test that measures the levels of certain proteins and hormones produced by the fetus. These include alpha-fetoprotein (AFP), human chorionic gonadatrophin (HCG), and estriol. The newest variation, known as a Quad screen includes these three proteins and another protein, inhibin, for a total of 4 serum analytes.
The screening tests may be abnormal if the level of AFP is too high. This raises the possibility that the fetus may have an abnormality of its spine, spina bifida. On the average, only 3-5 babies out of 100 whose mothers have a high AFP actually have a birth defect.
The screening test may also suggest an increased risk for Down syndrome. This typically results when your AFP level is low and your HCG level is high. Again, only a small percentage of women who have this result actually have a baby with Down syndrome. The test results predict a chance based upon your age and test results, but on the average only 1 woman in 150 who have an abnormal result actually have a baby with Down syndrome. In this situation, an amniocentesis can tell your baby's chromosome results.