ABSTRACT. Surfactant proteins A and B (SP-A and SP-B) were measured in human amniotic fluid by ELISA and correlated with lecithin to sphingomyelin ratio (L/S), phosphatidylglycerol (PG), and perinatal outcome. Amniotic fluid SP-A, SP-B, and L/S increased with advancing gestation. SP-A was detected at 19 wk gestation and increased dramatically in the 3rd trimester of pregnancy. SP-B was first detectable at 31 wk gestation and increased significantly to term. SP-A was a more specific predictor of nonrespiratory distress syndrome (RDS) than L/S or SP-B; however, the sensitivity of SP-A in predicting RDS was less than L/S < 2.0 (26.3 versus 82.3%, respectively). In 209 pregnancies assessed within 48 h of delivery, the sensitivity of SP-B in predicting RDS (nondetectable SP-B) was comparable to the L/S, however, SP-B = 0 was frequently observed in mature infants, limiting its specificity for prediction of RDS. The greatest sensitivity and specificity were achieved with the measurement of L/S < 3). SP-B is an M, = 8700 hydrophobic polypeptide that imparts biophysical properties of rapid spreading and adsorption to surfactant phospholipids (for review, see Ref. 4). The surfactant phospholipids and associated proteins are developmentally regulated, appearing in the fetal rat lung between 18 and 19 d of gestation and increasing until birth (5-7). Lung fluid, including surfactant, contributes to the composition of amniotic fluid. Amniotic fluid content of saturated phosphatidylcholine (lecithin), PG, and phosphatidylinositol parallel the expression of these phospholipids in the fetal lung and have been used to estimate fetal lung maturity (8-13). Human amniotic fluid content of SP-A also increases with advancing gestation and is predictive of fetal lung maturity and development of .RDS continues to be a major cause of morbidity and mortality in premature infants. Accurate estimates of fetal lung maturity are required for optimal pennatal-neonatal management of highrisk pregnancies. The L/S and the presence of P C are routinely used to predict the development of RDS (9, 1 1, 14). The accuracy of the combined L/S and P C tests is approximately 90-95%. A high incidence of false-negative results occurs particularly with the L/S, such that fetal lung immaturity is predicted in many cases where neonatal pulmonary function is mature. In the present study, concentrations of SP-A and SP-B in human amniotic fluid were determined by capture ELISA. The predictive value of the amniotic fluid concentration of SP-A and SP-B for neonatal pulmonary disease was compared with that of the L/S and PC. MATERIALS AND METHODSAmniotic fluid samples (n = 473) were obtained from 408 high-risk pregnancies at the University of Cincinnati between

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