The lecithin/sphingomyelin (L/S) ratio in amniotic fluid is widely used to predict the risk of respiratory-distress syndrome. However, the results are unreliable if the specimen is contaminated or obtained during a complicated pregnancy. We therefore compared the predictive value of the L/S ratio with that of the concentration of saturated phosphatidylcholine (SPC) in 322 amniotic-fluid samples, 75 per cent of which were contaminated or obtained during complicated pregnancies or both. A positive result is one that predicted the development of respiratory-distress syndrome, taken as an L/S ratio equal to or less than 2/1 or an SPC below 500 μg per deciliter. The respiratory-distress syndrome was correctly predicted in 25 of 45 cases (55.5 per cent) with L/S ratios equal to or less than 2/1, and in 35 of 42 cases (82 per cent) with SPC's less than 500 μg per deciliter. When L/S ratios were greater than 2/1, there were 13 of 277 (4.7 per cent) false negatives, and when SPC's were above 500 μg per deciliter, there were three of 280 (1.1 per cent) false negatives. We conclude that determination of SPC is both more specific and more sensitive as a predictor of the respiratory-distress syndrome than the techniques currently in use. (N Engl J Med 301:1013–1018, 1979) THE use of the lecithin/sphingomyelin (L/S) ratio in amniotic fluid as a method to predict the risk of respiratory-distress syndrome was a major discovery in perinatology, with immediate widespread acceptance. Shelley et al.1 have recently called attention to the fact that current biochemical tests for fetal lung maturity in clinical use lack specificity for pulmonary surfactant. Ogawa was one of the first to attempt to measure surfactant lecithin* in sheep amniotic fluid and relate it to fetal lung maturity.2 Others have determined foam stability,3 surface tension,4 the ratio of palmitic to stearic acid,5 surfactant apoprotein6 or dipalmitoyl lecithin,7 in an.
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