A modified procedure for evaluation of the lecithin-sphingomyelin ratio in amniotic fluid.

We describe several modifications of the procedure originated by Gluck et al. [Amer. J. Obstet. Gynecol. 109, 440 (1971)] for evaluation of amniotic fluid lecithin/sphingomyelin (L/S) ratios. The acetone-precipitation step is eliminated, resulting in a faster extraction procedure and higher concentrations of phospholipids. For faster thin-layer chromatography, we use silica gel-impregnated sheets of glass-fiber and a modified solvent system. For spot detection we use a noncorrosive bismuth subnitrate reagent that is specific for lecithin and sphingomyelin, and an iodine vapor method to confirm the L/S ratio. The bismuth spray also detects significant specimen contamination by phospholipids from plasma or erythrocytes. The L/S ratio is evaluated by visually comparing the relative size and color intensity of the lecithin and sphingomyelin TLC spots. An L/S ratio of 5 or more is consistent with mature pulmonary function and an L/S ratio of less than 2 suggests fetal pulmonary immaturity.