False positive amniotic fluid alpha fetoprotein levels resulting from contamination with fetal blood: results of an experiment.

The finding of an elevated level of alpha fetoprotein (AFP) in amniotic fluid is of value in the prenatal diagnosis of open neural tube defects. The present study was done to determine the amount of fetal blood required in amniotic fluid to produce a significant and misleading increase in AFP. Fetal blood was obtained at hysterotomy, and measured volumes were added to amniotic fluid samples. Bethe-Kleihauer tests, red cell counts, and AFP determinations were done. On the average, at 16 weeks' gestation, contamination of 5 ml. of amniotic fluid with 22 mul of fetal blood will results in an increase in AFP of 1.6 mg. per deciliter. Fetal cells in a much lower concentration can readily be detected by the Bethe-Kleihauer technique. A Bethe-Kleihauer test and red cell count should be done on all blood-stained amniotic fluid samples to determine the amount of fetal blood present. The contribution of the fetal blood AFP can then be estimated and must be considered in the interpretation of the total amniotic fluid AFP result.