Sedimentation of lung-derived phospholipid during low-speed centrifugation of amniotic fluid

In most methods proposed for the assessment of fetal lung maturity, amniotic fluid is subjected to a preliminary low-speed centrifugation in an attempt to separate whole cells and cell debris from lung-derived surfactant phospholipid (lamellar body phospholipid). However, because lamellar body phospholipid is present in amniotic fluid in a membranous or particulate form, it is also partly sedimented by this procedure. The sedimentation of total phospholipid and lamellar body phospholipid by low-speed centrifugation has been quantitated for 49 samples of amniotic fluid from pregnancies of 30--41 weeks gestation. Isopycnic density-gradient centrifugation in a small air-driven ultracentrifuge was used to isolate lamellar body fractions from whole and centrifuged amniotic fluid. Centrifugation for 5 min at 1000 x g removed 0--70% of total phospholipid or lamellar body phospholipid, the mean values being 34 or 29%, respectively. There was an appreciable increase in lamellar body phospholipid relative to total phospholipid as a result of centrifugation in only 51% of the samples. In general, the effects of centrifugation were not related to gestational age of the fetus or the state of maturity of its lungs.