Abstract

We describe a new and specific method for measurement of lecithin palmitic acid in amniotic fluid. Dipalmitoyl lecithin, the major alveolar surfactant, has previously been estimated by measuring the lecithin-sphingomyelin ratio, total lecithin, total phospholipid phosphorus, and (or) total palmitic acid. Our method is more specific for estimation of dipalmitoyl lecithin, because nonphospholipid sources of palmitic acid are removed by solvent extraction. Using a hexane/2-propanol/sulfuric acid system, we obviated the major interferences from triglycerides and free fatty acids. The palmitic acid derived from the phospholipid fraction is measured by gas-liquid chromatography of its methyl ester. No contribution appears to be made by sphingomyelin palmitic acid--probably owing to the mild hydrolysis conditions. The measured palmitic acid therefore appears to be derived from lecithins, principally dipalmitoyl lecithin. The value for palmitic acid determined by this method correlates well with the lecithin-sphingomyelin ratio and total phospholipid phosphorus. Infants are unlikely to develop respiratory distress syndrome when the measured palmitic acid in amniotic fluid exceeds 8.0 mg/liter, which corresponds to an lecithin-sphingomyelin ratio of 2.0.