Quantified scoring of the amniotic fluid surfactant foam test and assessment of factors contributing to false negative test results

Surfactant foam test scores were quantified by measuring the percent ring (%R) of foam produced at the meniscus of the 1:1 dilution tube. A comparison of %R scoring with conventional negative-positive rating of the foam test indicated that the %R value provided a more accurate estimate of the lecithin/sphingomyelin ratio. Based on clinical assessment of the newborn infant, the %R score also reduced the incidence of false negative test results by 47%. Variations in the assay ethanol fraction or tube size, as well as low amniotic fluid lecithin concentrations, were found to be potential determinants of false negative foam test results. The incidence of these false negative findings can be reduced by use of a %R score that has been empirically calibrated against neonatal outcome.