Comparison of Visualization Methods Used to Measure the Lecithin/Sphingomyelin Ratio in Amniotic Fluid

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Abstract
Amniotic fluid lecithin/sphingomyelin ratios are commonly determined after these phospholipids are separated by thin-layer chromatography and made visible by one of several methods. Here, we comprehensively survey the experiences of other authors with these visualization methods, and we have evaluated the available techniques. We report standard curves of dipalmityl lecithin and sphingomyelin made visible by six of these methods. Bromthymol blue in ammonia vapor or charring with either sulfuric acid:water (1:1 by vol) or potassium dichromate (10 g/liter) in sulfuric acid:water (7:3 by vol) produces spots of similar intensity for both phospholipids. However, iodine vapor, a phosphomolybdic acid dip, or charring with ammonium sulfate, produces more intense sphingomyelin spots. We recommend that (a) visualization methods be used that produce similar spot intensities for both phospholipids so that the ratio determined reflects the actual relative amounts of lecithin and sphingomyelin, and (b) if clinical predictions are made based on interpretation tables provided in the literature, the visualization method on which the ratio interpretation tables are based gives results that correlate with the actual ratio.

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