Lecithin/sphingomyelin ratio versus rapid surfactant test in normal and diabetic pregnancies

Lecithin/sphingomyelin (L/S) ratio was measured and rapid surfactant test (RST) was performed on amniotic fluid samples drawn from 60 normal and 18 diabetic pregnancies, for determination of fetal lung maturity. The results were compared to the neonatal outcome. Comparison of L/S ratio and RST demonstrates a good correlation (95%) between the two tests in normal pregnancies. In diabetic pregnancies, however, correlation is notably lower (54%). Respiratory distress syndrome (RDS) was predicted in both L/S ratio and RST in 35-40% of normal pregnancies, with 60-65% of false negative results. In diabetic pregnancies the L/S ratio was as reliable as in normal pregnancies, but the RST was less reliable, with an RDS prediction rate of 18%. The assumption is made that increased amniotic fluid volume in diabetic pregnancies, results in dilution of the surfactant giving more false negative results in RST than are seen with L/S ratio, as in the latter, by measuring the ratio between the two constituents of amniotic fluid, the dilution factor is abolished. In diabetic pregnancies, a positive RST is reliable, but with negative results, L/S ratio must be determined for correct assessment of fetal lung maturity.