Surfactant is a complex mixture of compounds, with a significant amount of phospholipid. Lecithin is one of the primary phospholipids present, and its concentration in the amniotic fluid increases as the fetus matures. Sphingomyelin is another phospholipid present but its concentration tends to remain constant during the third trimester. It can therefore be used to identify the relative amount of lecithin present. The ratio of lecithin-to-sphingomyelin (the L/S ratio) of the amniotic fluid is used to assess the fetus for lung maturity which is important in gauging the risk for the respiratory distress syndrome in the infant after delivery.


Specimen: Amniotic fluid, collected in siliconized tubes and protected from light. The test can also be done on a gastric aspirate collected from an infant within 6 hours of birth, provided no milk has been given.


Testing: Thin layer chromatography with determination of the relative amounts of lecithin and sphingomyelin.


L/S ratio =

= ((relative amount lecithin) / (relative amount sphingomyelin))



• The L/S ratio is 1.0 at 32 weeks and reaches 2.0 at 35 weeks of gestation.

• If the L/S ratio is greater than 2:1 and phosphatidylglycerol (PG) is present, then the lungs are mature and delivery can be undertaken.

• If the L/S ratio is less than 2:1 and PG is absent, then the lungs are immature and respiratory distress is likely if immediate delivery is undertaken.

L/S Ratio


< 1.0


1.0 - 1.5


1.5 - 1.9


2.0 - 2.5

mature but caution

2.5 - 3.8


> 3.8

post term


False positive for "maturity" (L/S > 2, but infants develop respiratory distress) may occur:

(1) in diabetic mothers (common)

(2) contamination of amniotic fluid with blood

(3) erythroblastosis fetalis

(4) occasionally with other fetal or placental disorders



(1) blood

(2) vaginal mucus

(3) meconium

(4) vernix


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