Rational basis for foam-stability assays of amniotic fluid surfactant.

We evaluated the relative contributions of various phospholipids to the formation of foam in the amniotic fluid "foam stability test", by use of an artificial system of saline, ethanol, and dipalmitoyl 3-sn-phosphatidylcholine to determine the relationship between dipalmitoyl 3-sn-phosphatidylcholine concentration, ethanol volume fraction, and the threshold of formation of stable foam. At dipalmitoyl 3-sn-phosphatidylcholine concentrations of 20, 40, and 80 mg/liter, the threshold ethanol volume fraction was 0.46, 0.48, and 0.51, respectively. We similarly evaluated the ability of other phospholipids to form stable foam at various concentrations and ethanol volume fractions and found: bovine brain sphingomyelin greater than dipalmitoyl 3-sn-phosphatidylcholine greater than egg sphingomyelin greater than egg lecithin greater than phosphatidylglycerol. The corresponding propensities of different chemical species of phosphatidylcholine to form foam at 40 mg/liter were: dipalmitoyl 3-sn-phosphatidylcholine greater than dioleoyl phosphatidylcholine greater than dimyristoyl phosphatidylcholine greater than dilauroyl phosphatidyl-cho-line.