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 L/S 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 L/S ratio and LBC. There were significant relationships between L/S 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 L/S 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 L/S 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 L/S 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 L/S > 2 or LBC > 30,000.