Improved procedure for lecithin/sphingomyelin ratio in amniotic fluid reduces false predictions of lung immaturity.

Many laboratories have found that their procedure for determining lecithin/sphingomyelin (L/S)ratios gives an unacceptably high proportion of false predictions of fetal lung immaturity. We investigated each step in the procedure, in an attempt to improve the clinical performance of the test and to make the method more amenable to standardization between laboratories. L/S ratios were determined by the new procedure in amniotic fluid from 147 pregnancies, collected within two days of delivery of the infant. Four cases had an L/S ratio less than 2; all developed hyaline membrane disease. No other cases of hyaline membrane disease were encountered in this study. For 106 of these pregnancies, the L/S ratio was also determined by the procedure previously in use. This predicted lung immaturity for 16 infants, only four of whom developed hyaline membrane disease. All 12 cases incorrectly predicted as immature by the old procedure were correctly classified by the new procedure.