The lecithin- Sphingomyelin Ratio or L/S Ratio Is A Test Of Fetal Amniotic Fluid To Assess For Fetal Lung Immaturity.
The lecithin- Sphingomyelin Ratio or L/S Ratio Is A Test Of Fetal Amniotic Fluid To Assess For Fetal Lung Immaturity.
The ability of the fetus to survive extrauterine adaptation is greatly dependent on proper functioning of the pulmonary system. Infants born with respiratory difficulties are said to have respiratory distress syndrome (RDS). A deficiency of surfactant in the lungs is considered the primary etiological defect in RDS. Surfactant is a surface tension reducing substance essential to the function of the mature lung. It has three main functions: (1) to reduce surface tension so less pressure is required to hold the alveoli open; (2) to maintain alveolar stability by varying surface tension with alveolar size; (3) to inhibit the exudation of liquid from the pulmonary circulation into the airways. Surfactant is a complex mixture of lipids, proteins, and carbohydrates. In the mature lung 90-95% of the lipids are phospholipids, the major surface active component of which is dipalmitoyl lecithin (lecithin). The second major component is phosphatidylglycerol (PG), and other minor components include phosphatidylinositol (PI), phosphatidylethanolamine (PE), phosphatidylserine (PS), lysolecithin, and sphingomyelin.
In early pregnancy the concentration of lecithin in amniotic fluid is very small, comprising about 21% of the total lipids at the 20th week. The proportion of sphingomyelin at the time is 51.0%. As pregnancy progresses, the sphingomyelin level remains fairly constant while the lecithin level continues to increase, showing a very sharp increase after the 35th week. In the mature lung, lecithin comprises from 50 to 80% of the total surfactant lipids. The consistent level of sphingomyelin
provides an internal standard for comparison with the surface active lecithin, thus providing the basis for the L/S ratio test.
Since the introduction of Gluck's two dimensional thin layer chromato-graphic procedure in 1971,1 the L/S ratio has gained acceptance as the most reliable prognostic index of fetal maturity. The critical values of L/S ratios as related to fetal lung maturity are divided into three cate-gories: immature, transitional and mature. No exact borders exist between these phases and test results must be interpreted carefully. Although a "mature" L/S ratio reliably indicates fetal lung maturity in
most cases, it must be interpreted with caution in certain complications of pregnancy, some of which accelerate lecithin production while others retard it.2 In such cases, use of a battery of tests is recommended.3-9 Recent investigations indicate that two other phospholipids, phosphati-dylglycerol and phosphatidylinositol, are important to surfactant activity. 10-13 In normal pregnancies, PI increases after 30 weeks of gestation, peaks at 36-37 weeks, then gradually decreases. 10
PG increases after 35 weeks and continues to increase beyond term. 10 While research indicates that PI is an important surfactant component urther studies are needed to determine its exact role. 8,12 Careful review of the literature provides three important facts concerning PG: (1) in the majority of cases in which PG was present, infants (some of which had an immature or borderline L/S ratio) have not suffered RDS; (2) a small number of cases have been reported in which PG was present and the infants did suffer RDS; (3) the absence of PG does not necessarily mean that RDS is inevitable. 10-12,14 The presence of a "pseudo" PG band has been documented in some
cases. 15,16 This band, migrating very close to or in the PG position, may be eliminated by modifying the Helena procedure. 15,16 The Helena Fetal-Tek 200 Method provides a one dimensional thin layer chromatographic technique for separating phospholipids in amni-otic fluid. The test, run on a single plate, provides excellent resolution of lecithin and sphingomyelin because the PI, PG, PE and PS migrate far ahead of the lecithin and sphingomyelin. Other modifications of the original Helena L/S Ratio Test include a longer plate with a pre-adsorbent area in addition to the adsorbent area, and a new solvent system. 17 The preadsorbent area on the plate facilitates sample appli-cation since the sample can be applied rapidly as a streak across the TLC channel. Application of the sample to the preadsorbent layer
greatly improves resolution of the phospholipid bands
About the Author:
L/S ratio - Helena Laboratories is a clinical laboratory instrument and reagent manufacturer. Our clients include major medical centers, small hospitals, large reference laboratories and small private doctor's laboratories.
Contact Details:
HELENA Laboratories
Beaumont, Texas,
USA
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Categories: Health
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