Amniotic fluid absorbance at 650 nm: Its relationship to the lecithin/sphingomyelin ratio and neonatal pulmonary sufficiency

In the present study, we sought to assess the clinical efficacy of the optical density of amniotic fluid at 650 nm (A₆₅₀) to predict lung maturity in the human fetus. The A₆₅₀ of 113 samples of amniotic fluid obtained from 16 to 45 weeks' gestation was determined. Mature values (A₆₅₀ ≥ 0.11) were not observed until 35 weeks' gestation but were always present after 39 weeks' gestation. In those infants delivered within 48 hours of amniocentesis, the absence rates of respiratory distress syndrome were the same with a mature lecithin/sphingomyelin (L/S) ratio (97.9%) and a mature A₆₅₀ (97.3%). However, immature values in both tests were poor prognosticators of respiratory distress syndrome, with a rate of 37.5% with the immature L/S ratio and 15.8% with the immature A₆₅₀. A mature A₆₅₀ may be substituted for the L/S ratio, but an immature A₆₅₀ is less reliable. In addition, we found that differences in centrifugation altered the A₆₅₀ value, whereas exposure to light and cold storage did not.