Sampling of amniotic fluid for L/S ratio determination after rupture of membranes

After rupture of membranes (RM), amniotic fluid (AF) leaking from the external os can be collected by means of a sterile bladder catheter inserted into the vagina and connected to a plastic bag for determination of the L/S ratio. This study was designed to ascertain whether contamination of AF with cervical or vaginal mucus alters the L/S ratio determined by Gluck's method. 50 gravidas in term or preterm labor had their amniotic sac punctured by a long gauge-20 needle under amnioscopic sight. Two samples of AF were collected from the needle. After RM, a sterile plastic bladder catheter was placed in the vagina and connected to a plastic bag ordinarily used for urine collection. Two additional samples of AF were taken from the plastic bag. All 4 samples were marked with code numbers and submitted for L/S ratio determination. There were no significant differences in the L/S ratio between samples obtained by the two different techniques. It is concluded that sampling of AF by means of a vaginal catheter and a plastic bag does not alter the L/S ratio, and is therefore practicable for assessing fetal lung maturity after RM.