Value of glial fibrillary acidic protein determination in amniotic fluid for prenatal diagnosis of neural tube defects.

Glial fibrillary acidic protein (GFAp), an intracellular protein specific to astrocytes of the central nervous system, was determined by 2-site immunoradiometric assay in amniotic fluid from 78 pregnancies with a normal, and 100 with an abnormal outcome. GFAp was not detectable in any of the normal pregnancies, but there were measurable, and so raised, levels in 23 out of 25 cases of anencephaly and 4 out of 7 cases of spina bifida, which therefore allowed prenatal diagnosis. GFAp was also increased in 4 of 6 cases of fetal intrauterine death, but not in other congenital malformations associated with elevated alphafetoprotein levels or abnormal acetylcholinesterase banding pattern, such as exomphalos, other gastrointestinal malformations or renal abnormalities. GFAp is therefore specific for diagnosing open neural tube defects. The determination of GFAp in amniotic fluid was slightly less efficient overall than AFP for the prenatal diagnosis of neural tube defects, but can be a useful ancillary test and has the advantage of specificity.