MSAFP and Multiple Marker Screening

While amniocentesis for Down syndrome is perhaps better known, the test which truly revolutionized prenatal diagnosis was maternal serum alpha fetoprotein (MSAFP) screening, which became the first screening test offered to all pregnant women solely for the purpose of discovering risk for a fetal anomaly.

MSAFP screening was developed to detect neural tube defects (NTDs) in the fetus. NTDs comprise a set of defects involving the development of the brain and spine and leading to varying degrees of physical and cognitive impairment, some of which are incompatible with life; they are among the most common of serious birth defects. Finding fetal NTDs is complicated by the fact that over 90 percent occur to women at no known risk, making it necessary to offer testing to the entire population of pregnant women to detect any reasonable percentage of fetal NTDs.

Alpha fetoprotein is a substance produced by the developing fetus and present in maternal blood during pregnancy. In the early 1970's, it was found that higher than normal levels of MSAFP correlated with increased risk of fetal NTDs. This suggested the possibility of an inexpensive, minimally invasive, screening modality for NTDs (Brock, Bolton, and Monaghan).

In the 1980's, researchers linked lower than normal levels of MSAFP to Down syndrome and other chromosomal abnormalities, thus expanding the utility of the test (Merkatz, Nitowsky, Macri, et al.). Early pilot projects demonstrating the feasibility of MSAFP testing increased enthusiasm for it as a prenatal screening test, and the screening became firmly established as standard of care in the United States when an American College of Obstetrics and Gynecology "Legal Alert" warned obstetrical providers that failure to offer the test might leave them open to liability in the case of a baby born with a detectable anomaly (ACOG, 1985).

However, one concern about using MSAFP to detect Down syndrome was that it had much lower sensitivity and specificity for chromosomal abnormalities that it did for NTDs. When it was found that the addition of other biochemical markers improved the ability of the screen to predict Down syndrome, these quickly became added to the analysis. Most providers perform multiple marker screening, with a triple marker screen including human chorionic gonadotrophin and unconjugated estriol being the most common. Since all these analytes are gathered from the same

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