Modified method for estimating the phosphatidyl choline: sphingomyelin ratio in amniotic fluid, and its use in the assessment of fetal lung maturity.

Phospholipids of amniotic fluid were extracted, separated by thin-layer chromatography, detected with acidified ammonium sulfate, and charred; the phosphatidyl choline and sphingomyelin fractions were quantitated by transmission densitometry.The densities were recorded as integrator trace units.We used commercially available precoated plates and a solvent system to eliminate the interference of phosphatidyl serine. The procedure used for visualization e’iminated the irritating and corrosive effects of the 50% sulfuric acid spray used in other methods. Omission of the acetone precipitation step decreased assay time without loss of accuracy. In 117 amniotic fluid samples obtained before deliveryof infantswho subsequently did not develop respiratorydistress syndrome (RDS) the geometric mean density ratio of phosphatidyl choline:sphingomyelin was 3.1, range 1.8to 8.6.Inthree pre-delivery amniotic fluid samples, densityratiosof lessthan 1.5were found. Those infants developed RDS.