Neutral-red uptake and expression of monocytic antigens in amniotic-fluid mononuclear phagocytes: evaluation of a novel approach for prenatal diagnosis of neural-tube defects.

In cases of fetal neural-tube defects macrophages are present in the amniotic fluid. We found that these viable phagocytic cells take up neutral-red and are easily identified as "red cells" by microscopic examination. This method is suitable for the rapid identification and counting of amniotic-fluid macrophages in suspension. We have studied 298 amniotic fluid samples. In the 226 normal cases studied, 0 to 1,200 macrophages per milliliter amniotic fluid have been found. In contrast, we found 1,250 to 99,000 macrophages per milliliter amniotic fluid in our 70 open neural tube defect (ONTD) cases. Statistical evaluation was performed to estimate the normal and pathologic ranges. Specificity and sensitivity of the neutral-red test and predictive value of positive and negative results have been calculated and presented in comparison with alpha-fetoprotein (AFP) determinations and ultrasonic methods. In 5 cases of anencephaly and 7 normal cases amniotic fluid cells were studied by immunocytochemistry: mononuclear cells present in the abnormal cases showed intense immunoreactivity for the Mo1 and Mo2 surface antigens of the phagocytic cell lineage.