In use now for nearly 20 years, MSAFP screening illustrates some of the dilemmas and potential complexities of determining increased risk of genetic disorders in the fetus (Brock and Sutcliffe, 1972). It is often the first screening test used to begin the process of identifying high-risk pregnancies, and can be followed by multiple prenatal diagnostic technologies, which may include further blood testing, ultrasound, and amniocentesis. An MSAFP test is a simple maternal blood test administered between 15 and 20 weeks of gestation as a first-step screening test to detect those pregnancies at possible increased risk for a variety of conditions; the MSAFP test does not diagnose fetal disorders.
The MSAFP screening test serves as a basis for appropriately referring, for more definitive tests, women who exhibit elevated or decreased levels of the fetal protein in their blood. High levels of AFP in the mother's blood might be associated with failure of the neural tube to close around the spinal column of the developing fetus. If the neural tube does not close completely, a variety of structural abnormalities may result, ranging from spina bifida (an open area on some part of the cover of the spinal column with varying degrees of disability) to anencephaly (the absence of the brain above the brain stem). The use of the MSAFP test for screening for increased risk of neural tube defects was approved by the U.S. Food and Drug Administration (FDA) in 1983. Although use of MSAFP for screening for increased risk of fetal Down syndrome (Cuckle and Wald, 1984; Merkatz et al, 1984; Hook, 1988) has also become widespread, this application of MSAFP testing has never been approved by the FDA (see Chapter 3). In 1985, the American College of Obstetrics and Gynecology distributed a professional liability "alert" advising obstetricians about the professional liability implications of AFP testing and advising them that "it is imperative" for them to discuss the availability of such screening with their patients and to document the discussion in the patient's chart (ACOG, 1985).
In addition, single and multiple fetal malformations, including heart, kidney, and gastric wall defects, or urinary and abdominal wall abnormalities, may be present when MSAFP is elevated (Crandall, 1992). An apparent abnormal MSAFP level may also result from misdating the pregnancy, multiple births, errors in determining or reporting race/ethnicity, diabetes mellitus, and body weight in the mother—all of which require adjustment of MSAFP levels for correct interpretation. Errors in reporting such critical information by the referring physician can result in additional laboratory errors in interpreting MSAFP results (Holtzman, 1992).
Several concerns have been raised regarding the routine use of MSAFP in prenatal care. ASHG has issued a policy statement recognizing that MSAFP testing was becoming part of routine obstetrical practice but stating that the test was not simply an office test (ASHG, 1987, p. 75). Among the concerns identified were (1) the complexity of setting cutoff levels for interpreting results due to the high rate of false positives and false negatives, doubts about the predictive value of the test results, and wide variability in the severity of the disorders for which