Amniotic fluid optical density: relationship to L:S ratio, phospholipid content, and desquamation of fetal cells

The relationship between amniotic fluid optical density at 650 nm (OD650) and the lecithin:sphingomyelin (L:S) ratio was compared in 174 samples of amniotic fluid from 143 patients. An OD650 greater than 0.15 accurately predicted and L:S ratio greater than 2.0 with an overall reliability of 67.9%. There were 19.5% false-negative and 12.6% false-positive predictions. The OD650 was found to have close correlation with the number of fetal squames in suspension in amniotic fluid. Pure surfactant phospholipid standards were found to exhibit negligible absorbance at 650 mm. OD650 may be more closely related to fetal cutaneous maturity than to pulmonary surfactant content and bears only an indirect relationship with pulmonary preparedness for independent existence. The OD650 test does not appear to be sufficiently reliable and specific to replace phospholipid determination as a data phase upon which to make clinical decisions regarding fetal pulmonary surfactant maturity.