Assessment of a rapid test on amniotic fluid for estimating fetal lung maturity WILLIAM



















We have measured the absorbance of centrifuged amniotic fluids at 650 nm. and found reasonably good correlation between the absorbance and the L/S ratio. In 67 fluids studied, an abSorbanCe of greater than 0.100 predicted correctly an US ratio of 2 or more in g6 per cent of the cases. When the absorbance was less than 0.100,70 per cent of the fluks had an L/S ratio of less than 2. (AM. J. OBSTET. GYNECOL. 130: 225, 1978.)

We and others4 have observed that there is a relationship between amniotic fluid turbidity and the L/S ratio. The L/S ratio is generally higher for turbid fluids, and it is possible to quantitate turbidity by spectrophotometry. Sbarra and associates,5 measured the absorbance of amniotic fluid samples at 400 nm. and found in 24 samples that an absorbance of 0.28 or more correlated well with an L/S ratio of two or more. They had only one false negative, which had an absorbance of 0.25 and an L/S ratio of 2.8/l. No faisepositive values were reported. A serious disadvantage associated with using 400 nm. is that bilirubin, hemoglobin, and meconium absorb at this wavelength. An absorbance at 400 nm. could be due to any of these three substances and might not be due to the turbidity of the fluid. In an addendum, Sbarra and associates3 suggested using 650 nm., a portion of the spectrum away from the wavelengths where pigments absorb. We have attempted to repeat the o’bservations of Sbarra and associates.5 Their procedure is based on the absorbance of 650 nm. of centrifuged amniotic fluid.

THE LECITHIN-SPHINGOMYELIN (L/S)ratio as described by Cluck and associates.’ has been widely accepted for judging fetal lung maturity. An L/S ratio of 2 or greater indicated pulmonary maturity in the majority of cases and minimal risk of the newborn infant developing respiratory distress syndrome. L/S ratios of less than 2 are commonly found in cases showing the respiratory distress syndrome. Unfortunately, the estimation of the L/S ratio is technically difficult to perform and is not available in many smaller hospita1 laboratories. Considerable effort has been exerted to develop a simple and rapid test to judge fetal pulmonary maturity. Clements and associates2 developed the foam stability test for amniotic fluid in 1972. Amniotic fluid is considered to be mature if it can generate a stable ring of bubbles in the presence of alcohol after the fluid has been shaken for 15 seconds. This test is based primarily on the ability of the surfactant lecithin to generate a stable foam. The “shake test” has not been entirely satisfactory as the manipulations, cleanliness, tube diameter, etc., are critical in determining the final result. A report critical of the “shake test” has appeared in the literature.” From the Defartments cf Obstetrics Pathology, Ohio State University. Received jar publication .Iccepted


and Gynecology

Matwkl and lnahods Of 87 amniotic fluid samples examined, 84 were obtained by transabdominal amniocentesis from 75 patients, one was obtained from a vaginal pool, and two were obtained via aspiration through a uterine pressure catheter. Samples that contained blood or meconium, which could not be centrifuged into a precipitate, were discarded because of the abovementioned absorbance interference. Among the indications for amniocentesis were re-


Aug-ust 19, 1977.

20, 1977.

Reprint reqzwsts: Wtlliam Copeland, Jr., M.D., Department of Obstetrics and Gynecology, N645 LGversiQ Hospital, 410 W. 10th Ave., Columbus, J321O.




Copeland et al.

January Am. J. Obstet.

Table I. Agreement between absorbance at 650 nm. and L/S ratio for fluid obtained at or near term (N = 87) Arbitrary


cut off 0.050




Detwmined absorbance

LIS ratio

0.050 0.050 co.075 bO.075 co.075 >0.075 O.lOO O.lOO 0.150 co. 150 >0.150

Assessment of a rapid test on amniotic fluid for estimating fetal lung maturity.

Assessment of a rapid test on amniotic fluid for estimating fetal lung maturity WILLIAM COPELAND, JR., STEMPEL, M.D. LAURENCE JOHN A. WILLIAM...
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