Fetal lung maturity testing: what labs need to know now

Feb. 1, 2012

In the September 2009 issue of Medical Laboratory Observer, I had an opportunity to write about the laboratory assessment of fetal lung maturity (FLM). Now, only two-and-a-half years later, the landscape of FLM testing has changed significantly. Laboratorians and lab decision makers need to be aware of the changes.

As a quick reminder, respiratory distress syndrome (RDS) of the newborn is the seventh most common cause of perinatal mortality in the United States, and it is caused by a deficiency of pulmonary surfactant at birth. Pulmonary surfactants function to decrease the surface tension of lung alveoli so that they remain open and functional upon exhalation. If surfactant concentrations are too low, alveoli collapse occurs, leading to RDS. The incidence of RDS decreases with increasing gestational age; for example, its risk is greater than 60% at 29 weeks but less than 5% at 37 or more weeks.

Because fetal lung liquids contribute to amniotic fluid, the amount of surfactant in fetal lungs can be estimated by measuring the amount of surfactants in amniotic fluid. Over the last 40 years, several FLM tests were developed and used clinically for the assessment of fetal lung maturity. For many of those years, the most widely used FLM test has been the commercially available surfactant-to-albumin (S/A) ratio from Abbott Laboratories. As most laboratorians now know, Abbott retired the TDx and TDxFLx instruments that perform the S/A ratio and ceased manufacturing the reagent at the end of 2011. Recognizing its leadership position in FLM testing, Abbott provided advance notice of the retirement and provided information on other FLM testing options
(http://flmoptions.com). Other FLM tests still in use today include the lecithin-to-sphingomyelin (L/S) ratio, the qualitative detection of phosphatidylglycerol (PG), and the lamellar body count (LBC).

The impact that the loss of the S/A ratio will have on the laboratory and obstetric communities is only beginning to be felt, of course, and how significant that impact will be is still unknown. A recently published paper sought to determine how those two communities might adapt to its loss.1 A survey of obstetricians revealed that, without access to the S/A ratio, 68% would order the L/S ratio, 44% would order the LBC, and 28% would order the PG test. However, few laboratorians are prepared to meet what may be an increased demand for the L/S ratio, as only 18% of laboratories offer the test.

While requests for the L/S ratio could be addressed by sending out samples to a laboratory that offers the test, the surveyed physicians also indicated that they expect test results to be returned within 12 hours of sample collection, an expectation that would be difficult to meet for a referred test. Why do so few labs perform the L/S ratio? The primary reasons are that it is a lengthy and technically difficult test to perform; it requires considerable expertise; and is imprecise. Although the detection of PG by immunoagglutination is rapid and easy to perform, PG is a late biomarker of pulmonary maturity, and this limits its clinical utility.

The only remaining option is the LBC test, which is considered by many to be the most appropriate replacement test for the S/A ratio. The LBC is rapidly performed using an automated cell counter (ubiquitous in nearly every clinical laboratory) and, most importantly, the LBC performs as well as the S/A ratio as a predictor of fetal lung maturity.2 However, there are potential barriers to widespread laboratory implementation of the LBC. A 2009 survey of laboratorians indicated that only 13% currently offered the LBC test and that, of the 87% that did not, only 16% were planning to validate it for clinical use in their laboratory.1 A lack of familiarity with the test, a decreased demand for FLM testing, and the fact that the LBC was a lab-developed test were the top three reasons cited by survey respondents when they were asked why they had no plans to offer the LBC test. It is suspected, but not known for certain, that there has been wider uptake of the LBC test than would be predicted from the survey data, particularly as the retirement of the S/A ratio grew closer.

In 2010, recognizing that there may be increased interest in the LBC test, the Clinical and Laboratory Standards Institute approved the creation of a document to provide guidance for the use of automated cell counting to enumerate lamellar bodies in amniotic fluid. The final document, Assessment of Fetal Lung Maturity by the Lamellar Body Count (C58-A), was published in November 2011. It describes the different counting technologies used in automated cell counters, as well as methods laboratorians can use to verify/validate the LBC test. Key highlights in the document include providing guidelines for the use of automated cell counting to perform the LBC test; describing methods to assist in test verification and validation; and describing methods to select an appropriate maturity cutoff. The document3 serves as a useful reference for laboratories that choose to offer the test.

While laboratorians may be struggling with the loss of the S/A ratio and wondering what to offer in its absence, current evidence makes one wonder if FLM tests are clinically relevant anymore. It has been firmly established that delivery before 39 completed weeks of gestation is associated with greater risks of neonatal death from RDS compared to infants born later. A longstanding assumption has been that delivery before 39 weeks may be acceptable if there is biochemical evidence of fetal lung maturity. A recent paper by Bates and colleagues challenged that assumption and clearly demonstrated that even when fetal lung maturity was demonstrated, infants born before 39 weeks were significantly more likely to experience an adverse outcome (respiratory or otherwise).4

There is another factor to consider when addressing the relevance of FLM testing: due to improvements in gestational age dating, maternal administration of corticosteroids that accelerate fetal lung maturity in at-risk pregnancies, and exogenous surfactant replacement therapies, the number of newborn deaths due to RDS has continued to decline over the last 15 years. Interestingly, most laboratories have noted a decline in the number of FLM tests that they perform each year. This trend reflects the decreased use of the tests by obstetricians, many of whom indicate that the tests are no longer needed for patient care.1 When one considers these facts in light of the Bates study, it becomes legitimate—and provocative—to ask the question: “Are tests of fetal lung maturity obsolete?”

David Grenache, PhD, is associate professor of pathology at the University of Utah School of Medicine and medical director of the Special Chemistry Laboratory at ARUP Laboratories.
He is board-certified by the American Board of Clinical Chemistry and is a fellow in the National Academy of Clinical Biochemistry.
He also maintains a blog “The Pregnancy Lab” (www.pregnancylab.net), focused on the laboratory tests performed during pregnancy.


  1. Grenache DG, Wilson AR, Gross GA, Gronowski AM. Clinical and laboratory trends in fetal lung maturity testing. Clin Chim Acta. 2010 Nov;411(21-22):1746-1749.
  2. Haymond S, Luzzi VI, Parvin CA, Gronowski AM. A direct comparison between lamellar body counts and fluorescent polarization methods for predicting respiratory distress syndrome. Am J Clin Pathol. 2006 Dec;126(6):894-899.
  3. CLSI. Assessment of fetal lung maturity by the lamellar body count; approved guideline. Wayne, PA: Clinical and Laboratory Standards Institute; 2011. CLSI document C58-A.
  4. Bates E, Rouse DJ, Mann ML, Chapman V, Carlo WA, Tita ATN. Neonatal outcomes after demonstrated fetal lung maturity before 39 weeks of gestation. Obstet Gynecol. 2010 Dec;116(6):1288-1295.