Relationship between amniotic fluid optical density and L/S ratio.

Determination of the lecithin-sphingomyelin (L/S) ratio to predict fetal lung maturation requires rather complicated laboratory techniques which are not always available on a 24-hour basis in all hospitals. For that reason more simple tests have been developed. One of these tests, the determination of amniotic fluid optical density at 650 nm (OD 650), was initially reported to correlate well with the L/S ratio, but later studies gave varying and conflicting results. To assess the possible usefulness of measurement of amniotic fluid OD 650 we determined the correlation between L/S ratio and OD 650 in 90 amniotic fluid samples from 90 pregnant women. All samples were obtained by transabdominal amniocentesis between 26 and 39 weeks' gestation. Indications for amniocentesis were threatened premature labor (n = 49), fetal growth retardation (n = 25), and rhesus sensitization (n = 16). The OD 650 and the L/S ratio were determined in the same amniotic fluid sample using standard techniques. A statistically significant positive correlation was found between the OD 650 and the L/S ratio. Considering an OD 650 reading of 0.15 or greater and an L/S ratio of 2.0 or higher as indicative of fetal lung maturation, 25.5% of the OD 650 readings appeared to be false positive, and 28% were false negative as compared with the L/S ratio. It is concluded that the low predictive values of positive and negative optical density readings preclude the clinical application of this simple test.