Functional phospholipid/sphingomyelin ratio: an easy to interpret indicator of fetal lung maturity.

Data are presented which show that the functional phospholipid/sphingomyelin (FPL/S) ratio is a sensitive and specific prenatal indicator of fetal respiratory status. Phospholipids included in the ratio are acetone precipitable lecithin(L), phosphatidylglycerol(PG), phosphatidylethanolamine(PE), and sphingomyelin(S). These lipids have been shown to contribute to either the lowering of surface tension or to changes in microviscosity. Both of these properties contribute independently to the physiologic effectiveness of fetal lung surfactant. Although other phospholipid profiles have been described in the literature, the FPL/S ratio accounts for the major phospholipids which contribute to lung surfactant function with the added advantage of presenting the data in the form of a single easy to interpret number. Clinical correlates are also presented for the L/S ratio, fluorescence polarization assay (using 1,6-diphenyl-3,5-hexatriene), and absorbance at 650 nanometers.