Measurement of the production of surfactant is the most direct means of prenatally assessing fetal pulmonary maturity. We review assays which have evolved for measuring surfactant, classifying them into two general categories: biochemical quantitation and biophysical measurements. Biochemical quantitation assays include the amniotic fluid lecithin/sphingomyelin ratio and quantitation of lecithin and other surfactant phospholipids. Biophysical measurements include measurement of surface-tension-lowering ability of extracts of amniotic fluid lipid and evaluation of surface-tension-related properties such as foam stability and microviscosity. Assays of surfactant are subject to certain pre-analysis sources of variation over which the analyst has no control, such as variability in total in vivo amniotic fluid volume, incomplete in vivo mixing of surfactant with amniotic fluid, and presence of contaminating blood or meconium. We also examine other factors such as centrifugation speed and time, and storage of the amniotic specimen before analysis. These factors can dramatically affect analyses, and must be carefully controlled by the analyst. In general, both biochemical and biophysical approaches to surfactant analysis are useful diagnostically. When properly performed, both approaches give results that correlate well, both with each other and with clinical outcome. Because "mature" and "immature" values overlap, none of the assays can completely eliminate false predictions, whether of fetal maturity or fetal immaturity.

Measurement of amniotic fluid surfactant.

Respiratory distress syndrome occurs in infants born with immature lungs. The immature fetal lung lacks an adequate supply of surfactant, a phospholipid-rich substance which is produced in the type II cells of the alveolar epithelium. In the fetus, surfactant is secreted into the potential air spaces of the lung and passes into the amniotic fluid as gestation proceeds. It is now clear that most methods currently in use for assessing fetal lung maturity depend on the detection of a sudden release of surfactant into the amniotic fluid as the lung reaches a critical stage of maturity. These methods, which include the lecithin/sphingomyelin ratio, the lung profile, total phospholipid or lecithin concentration, fluorescence depolarization, lamellar body phospholipid concentration, and the "shake" test, are reviewed in the light of recent understanding of the nature of surfactant. In assessing each method, we have examined possible sources of error in performing the test in the laboratory, factors which could theoretically limit its ability to reflect the state of fetal lung maturity and current information regarding its reliability, in terms of clinical performance. Guidelines for future research in this area are also suggested.

Methods of Evaluating Fetal Lung Maturity

Amniotic fluid lecithin and its fatty acid composition in respiratory distress syndrome.

A new technique for scanning tubular thin-layer chromatograms

Clinical studies on lecithin/sphingomyelin ratio.

A new method was developed for determining the concentration of amniotic fluid lecithin. During the last few years this concentration has been used to predict the risk of respiratory distress syndrome. After extraction, amniotic fluid phospholipids were precipitated with cold acetone, and subjected to thin-layer chromatography. In a thin-layer chromatographic detector the lecithin was oxidized in situ to carbon dioxide, which was further reduced to methane and measured with a flame ionization detector. The results of analyses of 37 samples of amniotic fluid taken after 28 to 42 weeks of gestation are presented.

Determination of Amniotic Fluid Lecithin with a Gas Phase Thin-Layer Chromatographic Detector

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Determination of Amniotic Fluid Lecithin with a Gas Phase Thin-Layer Chromatographic Detector