The outward flow of pulmonary secretions from the fetal lungs into the amniotic fluid maintains the level of lecithin and sphingomyelin equally until 32–33 weeks gestation, when the lecithin concentration begins to increase significantly while sphingomyelin remains nearly the same.

As such, if a sample of amniotic fluid has a higher ratio, it indicates that there is more surfactant in the lungs and the baby will have less difficulty breathing at birth.

If preterm delivery is necessary (as evaluated by a biophysical profile or other tests) and the L–S ratio is low, the mother may need to receive steroids such as betamethasone to hasten the fetus' surfactant production in the lungs.

Thin layer chromatography (TLC) is performed on the supernatant, which separates out the components.

Lecithin and sphingomyelin are relatively easy to identify on TLC and the predictive value of the test is good.