The interpretation and significance of the lecithin/sphingomyelin ratio in amniotic fluid

The significance of the lecithin/sphingomyelin (L/S) ratio is well established. However, numerous variations in techniques are appearing, and certain issues about techniques have emerged; among these, acetone precipitation, phosphorus determination rather than densitometry or area measurement, measurement of total phospholipids rather than the L/S ratio, and determination of lecithin concentration rather than the L/S ratio are discussed. Acetone precipitation allows examination of the physiologic (surface-active) fraction of lecithin; this fraction also helps to evaluate the rapidly maturing lung and possibly prevent false predictions. Maturational acceleration of the L/S ratio may be secondary to chronic stress or to acute stress such as acute placental infarction or ruptured membranes. Further evidence is presented that there is continuity between the fetal lungs and amniotic fluid. General principles are presented as a guide in clinical interpretation of the L/S ratio.