The
lecithin–sphingomyelin ratio (aka L-S or L/S ratio) is a test of
fetal amniotic fluid to assess for fetal lung immaturity. Lungs require
surfactant, a soap-like substance, to lower the surface pressure of the
alveoli in the lungs. This is especially important for
premature babies trying to expand their lungs after birth. Surfactant is a mixture of lipids, proteins, and
glycoproteins,
lecithin and
sphingomyelin being two of them. Lecithin makes the surfactant mixture more effective.