Surfactant proteins in the diagnosis of fetal lung maturity. I. Predictive accuracy of the 35 kD protein, the lecithin/sphingomyelin ratio, and phosphatidylglycerol.

The concentration of the major surfactant protein with a molecular weight of 35 kD was determined in 469 amniotic fluid specimens from 284 pregnancies by the two-site simultaneous immunoassay with monoclonal antibodies. The predictive accuracy of the 35 kD protein was compared with that of the lecithin/sphingomyelin ratio and phosphatidylglycerol (the lung profile). Immature levels of 35 kD protein (less than 0.6 micrograms/ml) predicted 59% of all cases of respiratory distress syndrome (RDS) with an accuracy of 91%, and mature levels of 35 kD protein (greater than 3.0 micrograms/ml) predicted 68% of all infants who did not have RDS with an accuracy of 100%. The overall accuracy of the 35 kD protein in predicting the risk of developing respiratory distress syndrome was similar to that of the lung profile. In addition, testing with 35 kD protein improved the predictive value of an indeterminate lung profile (lecithin/sphingomyelin ratio of 1:1.9 and no phosphatidylglycerol) from 52% to 74%. The present results show that the lecithin/sphingomyelin ratio, phosphatidylglycerol, and 35 kD apoprotein have additive effects in improving the accuracy of the diagnosis of lung maturity.