Review Question - QID 106471

The L/S (lecithin:sphingomyelin) ratio is used to determine whether or not there is adequate surfactant production in the fetal lung. An L/S ratio < 1.5 in the amniotic fluid indicates an insufficient production of surfactant and is predictive of neonatal respiratory distress.

Premature infants and infants of diabetic mothers are at increased risk of developing neonatal respiratory distress syndrome (RDS), caused by incomplete lung maturation and surfactant deficiency. Surfactant deficiency leads to increased alveolar surface tension and collapse, causing neonatal RDS, which affects 50% of infants babies born at 26–28 weeks, and about 25% at 30–31 weeks. Neonatal RDS usually presents within minutes of life with tachypnea, cyanosis, grunting, and nasal flaring, all indicating respiratory compromise. Testing of the amniotic fluid can examine the lecithin:sphingomyelin ratio which indicates the maturity of the fetal lung. A ratio < 1.5 is predictive of neonatal RDS.

Hermansen and Lorah review the causes, diagnosis, and treatment of respiratory distress in the newborn. The state that RDS can occur in premature infants as a result of surfactant deficiency and underdeveloped lung anatomy. The authors further describe that prenatal administration of corticosteroids between 24 and 34 weeks gestation reduces the risk of neonatal RDS when the risk of preterm delivery is high.

Besnard et al. perform a meta analysis to determine and compare the diagnostic accuracy of the lecithin/sphingomyelin (L/S) ratio and lamellar body count (LBC) in the prediction of neonatal respiratory distress syndrome (RDS). They find that LBC to be a superior test and recommend replacing the L/S ratio as gold standard with the lamellar body count since the LBC is easy to perform, rapid, inexpensive, and available to all hospitals 24h per day.

Illustration A demonstrates the levels of lecithin and sphingomyelin during gestation. Note the increasing L/S ratio as gestation progresses.

Incorrect Answers:
Answer 1: A lecithin:sphingomyelin ratio > 1.5 would be indicative of sufficiently mature lungs to have a low probability of neonatal respiratory distress syndrome.
Answer 2: The lecithin:phosphatidylserine ratio is not a ratio used to diagnose and predict the severity of neonatal respiratory distress syndrome.
Answer 3: A lecithin:sphingomyelin ratio > 3.0 would be indicative of sufficiently mature lungs to have a low probability of neonatal respiratory distress syndrome.
Answer 5: The lecithin:phosphatidylserine ratio is not a ratio used to diagnose and predict the severity of neonatal respiratory distress syndrome.