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Lamellar bodies, concentrically layered "packages" of phospholipid that represent the storage form of surfactant, can be counted in the platelet channel of most electronic cell counters. The lamellar body count has been used for more than a decade and performs as well as traditional phospholipid analysis as an assay for evaluating fetal lung maturity. It is preferable to phospholipid analysis because it is rapid, objective, and inexpensive and can be performed in any hospital laboratory. The current methodologies for specimen preparation vary widely among laboratories, most notably with respect to centrifugation, resulting in differences in maturity cutoffs used. Our goal was to establish a consensus regarding a standardized methodology for the lamellar body count. Institutions that previously had published their results with lamellar body counts were invited to contribute. The consensus of the four participating institutions includes the following: centrifugation is not a necessary step and should be abandoned, maturity is suggested by a count of 50,000/microL or greater, and immaturity is suggested by a count of 15,000/microL or lower. As the lamellar body count gains wider acceptance as a primary assay for assessing fetal lung maturity, the test must be performed uniformly and accurately, given the implications of acting on a falsely negative test resulting from improper methodology.

To study the effect of fatty acid chain saturation on the fluorescence polarization assay as a measure of fetal lung maturity, we used purified phospholipids isolated from human amniotic fluid and various commercial phospholipids. We found that the fluorescence polarization value decreased as the concentration of unsaturated fatty acids increased. In contrast, the lecithin/sphingomyelin ratio increases with increasing amounts of saturated lecithin, produced as the fetal lung matures. Since only saturated lecithins are surface active, the two indices of fetal respiratory status must reflect different properties of lung surfactant.

As an index of fetal lung maturity, the ratio of total surface-active phospholipid (phosphatidylethanolamine + phosphatidylinositol + phosphatidyglycerol + lecithin) to sphingomyelin performs as well as or better than any other indicator that we tested, showing 92% sensitivity and 98% specificity. The sensitivity and specificity, respectively, of the other methods were: (a) lecithin/sphingomyelin ratio, 92% and 95%; (b) alkaline phosphatase/glutamyltransferase ratio, 100% and 49%; (c) microviscosity, 75% and 97%; and (d) adsorbance at 650 nm, 50% and 73%. The procedure requires 2 h or less and its simplicity allows it to be offered on a 24 h/day basis without necessitating special training.

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.