A simple and rapid determination of the lecithin/sphingomyelin (LS) ratio in amniotic fluids and in pharyngeal aspirates of newborn infants.

A simple, rapid method for the routine determination of the lecithin/sphingomyelin (L/S) ratio in amniotic fluid and/or in pharyngeal aspirate of the newborn has been developed. In 33 samples of amniotic fluid at various gestational ages, an L/S ratio corresponding to the fetal age was found: in 16 samples of biochemically immature amniotic fluid (gestation age less than 34 weeks) the mean of the ratio was 1.26 and in 17 samples of mature amniotic fluid (gestation age greater than 35 weeks) the mean value was 3.02. Pharyngeal aspirates of 55 non-selected newborn were examined by the same method. The L/S ratio of 39 infants (gestation age greater than 38 weeks) gave a mean value of 7.3 and that of 16 premature infants (gestation age less than 37 weeks) a mean value of 5.5. In none of these cases did RDS develop after birth. The results suggest that the method is useful for the determination in both amniotic fluid and pharyngeal aspirate.