Deficient lung surfactant apoproteins in amniotic fluid with mature phospholipid profile from diabetic pregnancies.

An enzyme-linked immunoassay to quantitate lung surfactant apoproteins (15 to 250 ng/ml) in human amniotic fluid is described. The immunoassay was used to quantify lung surfactant in 72 samples of amniotic fluid, for which lecithin/sphingomyelin (L/S) ratios, lecithin and phosphatidylglycerol concentrations, and foam stability indices were also available. The results obtained with the immunoassay were in general agreement with those of the other methods. Measurement of the apoproteins, however, may be a better predictor of fetal lung immaturity and of respiratory distress syndrome than the L/S ratio and the concentration of lecithin. This conclusion is based on the data obtained in the analyses of samples of amniotic fluid from four diabetic and five nondiabetic pregnancies, the infants of which developed respiratory distress. In all cases, the apoprotein concentration was less than 2.1 micrograms/ml, which indicated fetal lung immaturity. In six of these cases (one diabetic and five nondiabetic pregnancies), lung immaturity was also predicted on the basis of other tests. However, in three other cases of diabetic pregnancy, the L/S ratio and lecithin concentration falsely indicated lung maturity. In addition to its being an effective predictor of fetal lung maturity in diabetic, as well as nondiabetic, pregnancies, the immunoassay is better suited for clinical use because of its high specificity, sensitivity, and ease of performance.