MSU Department of Pediatrics & Human Development

Test Description:
Neural tube defects such as spina bifida or anencephaly result when the embryonic neural tube fails to close properly at approximately 4 weeks' gestation. Prenatal screening for an open neural tube defect (ONTD) is possible by measuring the level of alpha fetoprotein (AFP) in maternal serum. Screen positive results are defined as those with an AFP MoM of 2.20 or greater. The optimal time for ONTD screening is 16 to 18 weeks geatation, however, the screen can be interpreted between 15 and 22 weeks gestation. The detection rate for ONTD's is 85% with less than a 3% screen positive rate. Ultrasound dating (especially the use of biparietal diameter) increases the detection rate and lowers the screen positive rate. Obtaining a second specimen for repeat testing may be beneficial when the initial specimen has a slightly elevated AFP (below 3.5 MoM) and a gestational age early enough to allow time for appropriate follow-up (below 19 weeks gestation). Conditions other than an ONTD associated with positive screening results include a multiple gestation, recent fetal demise or other abnormalities (e.g., omphalocele, gastroschisis). Unexplained elevated AFP levels are associated with an increased incidence of poor pregnancy outcome (e.g., low birth weight, pre-term delivery). The Quadruple Test and Integrated Tests are screens that include MSAFP for ONTD screening. MSAFP screening alone should be offered to patients who have had a First Trimester Test or Chorionic Villus Sampling as these do not include screening or diagnostic testing for ONTD's.