Is the L/S ratio really a lecithin/sphingomyelin ratio?

Fetal lung maturity is commonly assessed by determining the ratio of lecithin/sphingomyelin in centrifuged amniotic fluid. In a variety of chromatographic systems currently used for the routine determination of the lecithin/sphingomyelin ratio, including the systems recommended in the original procedure, at least one and frequently two additional phospholipids, normally present in amniotic fluid, tend to chromatograph between or overlapping with lecithin and/or sphingomyelin. These phospholipids have been tentatively identified as phosphatidylserine and phosphatidylinositol. The extra phospholipids contribute significantly to the routine lecithin/sphingomyelin ratio with considerable variation between individual cases. Treatment of dried lipid extracts with cold acetone before chromatography, as suggested in the original lecithin/sphingomyelin ratio procedure, does not remove the interfering phospholipids.