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The L/S Ratio and Shake Test in Normal and Abnormal Pregnancies

From the Department of Obstetrics and Gynecology, Division of Maternal/fetal Medicine, and the Department of Biochemistry at the University of Tennessee Center for the Health Sciences, Memphis, Tennessee

Abstract

The "foam" or "shake" test for fetal lung maturity has been shown by many investigators to be a rapid, reliable, and reproducible method of assessing fetal lung maturity when compared to the lecithin/sphingomyelin (L/S) ratio in normal pregnancies. In abnormal pregnancies, however, the accuracy of both tests in predicting respiratory problems has been questioned. This study shows the results of both procedures in a large group of normal (279) pregnancies, as well as in patients with significant maternal and/or fetal disease (489). In the normal patients, a positive shake test did not correlate with the L/S ratio in 4% of the cases, whereas a negative shake test compared favorably with the L/S ratio in 88% of the cases tested. If certain diseases were present, however, there was a disparity of 13-24% when these two tests were compared. When these procedures were used to predict the occurrence of RDS in the normal patients they were accurate. In the pregnancies complicated by maternal and/or fetal disease, however, the accuracy rate fell for both the L/S ratio and the shake test. In this group, the false-negative results appeared to remain stable while the false-positive rate demonstrated a significant increase. From these data, the shake test appears to predict RDS very well in normal patients. Unfortunately, because the number of errors increases significantly for both assays in abnormal patients, the tests should be used in conjunction with other tests if severe maternal or fetal disease is present.