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Lecithin/Sphingomyelin Ratio Amniotic Fluid Test

Last updated June 3, 2016

What are other Names for this Test? (Equivalent Terms)

  • L/S Ratio Amniotic Fluid Test

What is Lecithin/Sphingomyelin Ratio Amniotic Fluid Test?

  • The ratio of lecithin to sphingomyelin is used as a measure of the health of a growing fetus
    • Lecithin is a type of fat produced by the body to lubricate the tiny cells of the lungs, called the alveoli
    • Sphingomyelin is also a type of fat. It is valuable for testing purposes, because it is produced by the developing fetus at a constant rate during pregnancy
  • Measuring the amount of lecithin relative to the amount of sphingomyelin in a fetus gives information about the maturation of the fetus and the potential for having hyaline membrane disease
  • Normally, lecithin significantly increases after week 35 of pregnancy. Because sphingomyelin remains constant, this should translate to an increase in the lecithin/sphingomyelin (L/S) ratio. If the ratio is not around 3 or 4 by week 35 of pregnancy, there is a danger of alveolar collapse, and thus lung collapse (or atelectasis) because of lecithin deficiency
  • The Lecithin/Sphingomyelin Ratio Amniotic Fluid Test determines the L/S ratio in a fetus. It is used to determine fetal maturity and assess the risk of a respiratory syndrome upon delivering the baby

What are the Clinical Indications for performing the Lecithin/Sphingomyelin Ratio Amniotic Fluid Test?

Following are the clinical indications for performing the Lecithin/Sphingomyelin Ratio Amniotic Fluid Test:

  • Uncertain gestation period
  • Determining the risk of neonatal respiratory distress syndrome prior to delivery

How is the Specimen Collected for Lecithin/Sphingomyelin Ratio Amniotic Fluid Test?

Following is the specimen collection process for Lecithin/Sphingomyelin Ratio Amniotic Fluid Test:

Sample required: Amniotic fluid

Process of obtaining an amniotic fluid sample (amniocentesis):

  • A local anesthetic is applied to the site of amniocentesis
  • A needle is inserted into the pregnant woman’s abdominal wall, through the uterus and into the amniotic sac
  • An ultrasound scan is used to guide the needle, as it punctures the sac away from where the baby is located
  • Approximately 20 mL of amniotic fluid is obtained and the needle withdrawn

Preparation required: No special preparation is needed prior to the test.

What is the Significance of the Lecithin/Sphingomyelin Ratio Amniotic Fluid Test Result?

Interpretations of the Lecithin/Sphingomyelin Ratio Amniotic Fluid Test value depend on how far the pregnancy has progressed. 

  • A low value (less than 4:1, or 15-21 mg/dL for lecithin and 4-6 mg/dL for sphingomyelin) after week 35 of pregnancy may point to a possible hyaline membrane disease prior to delivery
  • A lowered test value may indicate that the fetal lungs are not probably ready to breathe on its own. This may also predispose the newborn to breathing difficulties necessitating intense medical attention at birth

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and Relevant Useful Information:

  • No single test is available to determine fetal distress. The Lecithin/Sphingomyelin Ratio Amniotic Fluid Test is just one of several tests, including the amniotic fluid index, used for this purpose

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

What are some Useful Resources for Additional Information?

The following DoveMed website link is a useful resource for additional information:


Please visit our Laboratory Procedures Center for more physician-approved health information:


References and Information Sources used for the Article:

Kee, J. L. (2010). Laboratory and diagnostic tests with nursing implications (8th ed.). Upper Saddle River, NJ: Pearson.

Martini, F., Nath, J. L., & Bartholomew, E. F. (2012). Fundamentals of anatomy & physiology (9th ed.). San Francisco: Benjamin Cummings.

Williamson, M. A., Snyder, L. M., & Wallach, J. B. (2011). Wallach's interpretation of diagnostic tests (9th ed.). Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins.

Helpful Peer-Reviewed Medical Articles:

Besnard, A. E., Wirjosoekarto, S. A., Broeze, K. A., Opmeer, B. C., & Mol, B. W. J. (2013). Lecithin/sphingomyelin ratio and lamellar body count for fetal lung maturity: a meta-analysis. European Journal of Obstetrics & Gynecology and Reproductive Biology, 169(2), 177-183.

Kwak, H. S., Chung, H. J., Choi, Y. S., Min, W. K., & Jung, S. Y. (2015). Prediction of fetal lung maturity using the lecithin/sphingomyelin (L/S) ratio analysis with a simplified sample preparation, using a commercial microtip-column combined with mass spectrometric analysis. Journal of Chromatography B, 993, 81-85.

Lee, S. M., Romero, R., Park, J. S., Chaemsaithong, P., Jun, J. K., & Yoon, B. H. (2015). A transcervical amniotic fluid collector: a new medical device for the assessment of amniotic fluid in patients with ruptured membranes. Journal of perinatal medicine, 43(4), 381-389.

Houben, M. L., Nordkamp, M. J. O., Nikkels, P., van der Ent, C. K., Meyaard, L., & Bont, L. J. (2013). sLAIR-1 IN AMNIOTIC FLUID IS OF FETAL ORIGIN AND POSITIVELY ASSOCIATES WITH LUNG COMPLIANCE. IMMUNE REgULATION, 67.

Varner, S., Sherman, C., Lewis, D., Owens, S., Bodie, F., McCathran, C. E., & Holliday, N. (2013). Amniocentesis for fetal lung maturity: will it become obsolete?. Reviews in obstetrics & gynecology, 6.

Mahoney, A. D., & Jain, L. (2013). Respiratory disorders in moderately preterm, late preterm, and early term infants. Clinics in perinatology, 40(4), 665-678.

Tsuda, H., Kotani, T., Sumigama, S., Mano, Y., Kawabata, I., Takahashi, Y., ... & Kikkawa, F. (2015). Amniotic lamellar body count: predicting and distinguishing neonatal respiratory complications in twin pregnancies. Clinica Chimica Acta, 441, 75-78.

Fields, M., Towers, C. V., Howard, B. C., Hennessy, M. D., Wolfe, L., Weitz, B., & Porter, S. (2015). Accuracy of the lamellar body count in amniotic fluid contaminated by meconium. The Journal of Maternal-Fetal & Neonatal Medicine, 28(2), 146-148.

Verder, H., & Höskuldsson, A. (2016). U.S. Patent No. 20,160,103,143. Washington, DC: U.S. Patent and Trademark Office.

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: June 3, 2016
Last updated: June 3, 2016