Measurement of amniotic fluid surfactant.

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.