The L/S ratio in vaginally collected amniotic fluid: a misleading result in a breech presentation

A case is reported in which serial L/S ratios, determined from fluid samples collected vaginally after premature rupture of the membranes, led to a grossly erroneous conclusion of fetal pulmonary immaturity. The fetus was in breech presentation, and in retrospect several 'amniotic fluid' samples consisted largely of fetal urine. Determination of the creatinine concentration of suspect samples should permit distinction to be made between mixed amniotic fluid and fluid containing a disproportionately large amount of fetal urine when the possibility of this error exists.