Sensitivity and specificity of the L/S ratio in relation to gestational age

A review of the literature was made, and 762 titles relating to fetal lung maturity were screened. From these, 195 articles or abstracts on the L/S ratio were reviewed. Five articles met the following criteria: (1) L/S ratio by the method of Borer and Gluck including acetone precipitation; and silica-gel-H plates, (2) interval of the L/S ratio to delivery of 72 hours or less, (3) exclusion of diabetics (except for one patient with an L/S ratio greater than or equal to 2 whose baby developed hyaline membrane disease (HMD) who may have been diabetic), (4) HMD well defined and documented, and (5) a breakdown of the data by gestational age. From these five articles, the sensitivity, specificity, and predictive values were calculated for less than 34 weeks, 34 to 37 weeks, and greater than 37 weeks of gestational age. The sensitivity of the L/S ratio was good regardless of gestational age. The specificity was very poor before 34 weeks, somewhat better between 34 and 37 weeks, and best after 37 weeks. The predictive value of a negative test result was moderate before 34 weeks and became excellent after 37 weeks. The predictive value of a positive test result was moderate before 34 weeks and became very poor after 37 weeks. In conclusion, after 34 weeks, an L/S ratio of 2 or greater was a reliable indication of fetal lung maturity. However, an L/S ratio of less than two was only slightly better than an accurate assessment of gestational age at predicting HMD.