Definition
- 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.
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TABLE 16 " “54Values of L:S Ratio and Lung MaturityView LargeTABLE 16 " “54Values of L:S Ratio and Lung Maturity L:S Ratio Values in Some Laboratories Lung Maturity <1 <2.0 Very immature lungs (up to 30th wk of gestation); severe RDS is expected; lung maturity may require many weeks; do not resample before 2 wk 1.0 " “1.49 Immature lungs; moderate to severe RDS is expected; lung maturity may occur in 2 wk; resample in 1 wk 1.5 " “1.9 2.0 " “3.0 Lungs on threshold of maturity (within 14 d); mild to moderate RDS may occur. Test should be repeated in 1 wk ≥2 >3.0 Mature lungs (35th week of gestation); low incidence of RDS even if phosphatidylglycerol is absent. S/S = 80 " “85% Abundant lecithin with trace or no sphingomyelin Postmature lungs
Use
- Traditional biochemical test to measure fetal lung maturity
Interpretation
- Increased in mature fetal lungs (see Limitations and Table 16.54 for high ratios)
- Decreased in immature fetal lungs
Limitations
- 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)
- Erythroblastosis fetalis
- Possible exceptions
- Intrauterine growth retardation
- Toxemia of pregnancy
- Hydrops fetalis
- Placental disease
- Abruptio placentae
- Foam (shake) test