Assessment of fetal lung maturity: a comparison of the lecithin/sphingomyelin ratio and the tests of optical density at 400 and 650 nm.

Seventy-eight amniotic fluid samples were obtained by transabdominal amniocentesis and were analyzed for their L/S ratio and optical density at 400 and 650 nm. The L/S ratio was considered to be mature if the values were greater than 2.0. The optical density of the fluids decreased with prolonged refrigeration. With freshly centrifuged samples, the OD650 reading of greater than 0.15 gave the best correlation with the mature L/S ratio. There were 3.8% false positive results and 14.1% false negative results. The study confirms that the OD650 test on fresh amniotic fluid is a rapid and inexpensive way to determine fetal maturity. All samples with values of less than 0.15 must, however, also be tested for their L/S ratio since some of the infants with these values may also be mature.