Lecithin/sphingomyelin ratio and lamellar body count - What values predict the presence of phosphatidylglycerol?

OBJECTIVE: To assess which values of the lecithin/sphingomyelin (L/S) ratio and lamellar body count (LBC) optimally correspond to. the presence of phosphatidylglycerol (PG). STUDY DESIGN: A database of clear amniotic fluid specimens obtained by amniocentesis was accessed. Receiver operating characteristic curve analysis was performed for both LIS ratio and LBC to identify the optimal thresholds for predicting the presence of PG. RESULTS: One hundred eighty-eight patients were included in the analysis. "Trace" PG was more like "absent" PG in relation to both LIS ratio and LBC. There were significant relationships between LIS ratios and presence of PG (area under the curve =.909, P<.0001) and between LBC and presence of PG (area under the curve =.94, P <.0001). The positive predictive value of LIS ratio > 2 was 76%, and that of LBC > 30,000 was 82% in predicting the presence of PG. Optimal threshold values for the prediction of PG presence were > 3.0 for LIS ratio, which had a positive predictive value of 95% and false positive rate of 5%, and > 50,000 for LBC, with a 96% positive predictive value and 5% false positive rate. CONCLUSION: "Mature" results for PG in amniotic fluid are equivalent to an LIS ratio of >3.0 and LBC > 50,000 and help explain the later gestational age at the appearance of lung maturity by PG than by LIS > 2 or LBC > 30,000.