Use of the drop volume of amniotic fluid in predicting fetal lung maturity. Clinical experience.

Respiratory distress syndrome (RDS), resulting from inadequate amounts of alveolar surfactant, is a leading cause of neonatal mortality. This study analyzed the drop volume for predicting the risk of neonatal RDS by evaluating the surface tension of amniotic fluid. The drop-volume method quantifies the surface tension of amniotic fluid using the fact that the volume of a falling drop of liquid is proportional to the quantity of surface-active substances (surfactant) in the solution. Distilled water is used as a reference solution. The test is simple to perform and requires only two minutes and 2 mL of amniotic fluid. Amniotic fluid was obtained by amniocentesis from 120 pregnancies, ranging from 24 to 42 weeks of gestation, within four days of delivery. Specimens contaminated with meconium or hemolyzed blood or from patients with clinical chorioamnionitis were not used. The lecithin:sphingomyelin ratio and phosphatidylglycerol tests were also determined for 54 of the study cases. A mature drop-volume test result was associated with a healthy infant with no RDS 95% of the time (negative predictive value). Fifty-two percent of the healthy infants in the study were predicted by an arbitrary unit of time (S) value of less than or equal to 85.0 (specificity). Thirteen of the 16 infants who developed RDS (81%) were predicted by an S value of greater than or equal to 85.1 (sensitivity). Of those infants with an S value of greater than or equal to 85.1, 21% developed RDS (positive predictive value).