Some medical tests check for levels of a particular substance in a sample. Others check the level of one substance against the level of another. A lecithin-sphingomyelin ratio test is one of these techniques, and it compares two fatty molecules that are both found on the inside surface of the lungs. Typically, the test is used to check how mature an unborn baby's lungs are, as the ratio of the two change as the baby develops. Amniotic fluid inside the placenta contains enough of the substances for a test to compare, so this is commonly the sample used.
A human lung contains many tiny structures called alveoli. These are little empty sacs that swap waste carbon dioxide from the blood for the fresh oxygen from the air in the environment. Babies live throughout an entire pregnancy in a liquid-filled placenta, and do not need to use their lungs until they are born. Each baby develops over the pregnancy, from a fertilized egg to a full-term baby, and the lungs are no exception to this rule. Premature babies have not had all the growing time necessary to finish off the lung development, and so can suffer from breathing problems outside of the womb.
Each alveoli inside the baby's lung needs to be both supported and have flexibility to change shape as the lung expands and contracts. In order for the inside of the lung to be sufficiently strong and flexible, the inner surface is lubricated by fatty substances. Lecithin is the most important molecule to keep the lung stable, and sphingomyelin is also present.
As the baby continues to develop in the later stages of pregnancy, the levels of lecithin rise but sphingomyelin concentration tends to remain stable. Low levels of lecithin mean that the lungs are unstable. Comparing the lecithin-sphingomyelin ratio can give a doctor useful information as to the lung development of the baby, and how high the risk of breathing difficulties may be.
When the doctor knows that the mother may go into premature labor, or if the baby must come out before term, then a lecithin-sphingomyelin ratio test can be useful. Some of the fatty substances are present in the amniotic fluid, which is the fluid inside the placenta that the baby floats around in. This test involves inserting a needle through the mother's abdomen into the placenta, and it does carry some risk to the life of the baby. If a baby has breathing difficulties outside of the womb, the lecithin-sphingomyelin ratio test may also be performed, although this is less common.