A prospective evaluation of the lecithin/sphingomyelin ratio and the rapid surfactant test in relation to fetal pulmonary maturity.

Fetal lung maturity was correlated to the results of amniatic fluid analysis in 578 samples obtained over a 2 1/2 year interval. Lecithin/sphingomyelin (L/S) ratios obtained by planimetric and densitometric methods were compared to the rapid surfactant test (RST). Both the planimetric L/S ratio (P L/S) and the densitometric L/S ratio (D L/S) correlated positively in a highly significant fashion with the RST. Both RST and D L/S were reliable in predicting fetal lung maturity (99 to 100 per cent, respectively). The RST was the most reliable test for fetal lung immaturity (69 per cent). Therefore, this series indicates that the RST could be used as a primary method with the D L/S being useful as an additional indicator of fetal maturity when the RST was intermediate, or the sample contaminated with blood and/or meconium.