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Subject: Lecithin-to-Sphingomyelin (L:S) Ratio
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The L:S ratio is based on the observation that there is outward flow of pulmonary secretions from the lungs into the AF, and this changes the phospholipid composition of AF, thereby enabling indirect assessment of fetal lung maturity. The concentrations of L and S in AF are approximately equal until 32–33 weeks of gestation, at which time the concentration of L begins to increase significantly, whereas the S concentration remains about the same. The measurement of S serves as a constant comparison for control of the relative increases in L because the volume of amniotic fluid cannot be accurately measured clinically. This technique involves TLC after organic solvent extraction. It is a difficult test to perform and interpret. The presence of blood or meconium can interfere with test interpretation. Empirically, the risk of respiratory distress syndrome (RDS) is exceedingly low when the L:S ratio is >2.0.
Normal range: see Table 16.54.
Traditional biochemical test to measure fetal lung maturity
Increased in mature fetal lungs (see Limitations and Table 16.54 for high ratios)
Decreased in immature fetal lungs
Labor-intensive test offered by few laboratories
Offers no advantage over fluorescence polarization
Sensitivity of >95%, specificity of 70%
Blood and meconium contamination can affect result
Definite exceptions to prediction of pulmonary maturity with L:S ratio >2.0
Infant of a diabetic mother (L:S ratio >2.0 has been frequently seen in cases in which RDS developed)
Intrauterine growth retardation
Toxemia of pregnancy
Foam (shake) test