Category Archives: Respiratory Distress Syndrome

Respiratory Distress Syndrome

The dawn of obsolescence for fetal lung maturity tests

Just over 3 years ago, I blogged about the declining use of fetal lung maturity (FLM) tests and questioned their clinical usefulness. I wanted to revisit this same issue again because a recent report has validated my sentiments. Actually, there have been two recent reports on this topic but the authors on that second report include myself and my co-blogger, Dr. Ann Gronowski, so in the interest of impartiality, I won't discuss our article here.

Historically, FLM tests have been used to determine the maturity of fetal lungs for a wide variety of indications (see table below) with the intent to deliver the infant if lung maturity was demonstrated.

FLM test indications

The authors of the report observed an incredible amount of disparity between doctors at different centers in regards to the use of FLM tests for different indications. Further, they speculated (accurately, I might add) that this lack of consistency suggested that as obstetricians became more comfortable with performing amniocentesis, more amniocenteses were performed which, in turn, expanded the reasons for FLM testing! They point out that this could be a major reason why there was an increase in late preterm deliveries in the United States. It was a classic self-fullfilling prophesy: more amniocentesis for FLM testing lead to more non-term deliveries when fetal lung maturity was demonstrated.

The authors then point to studies that have clearly demonstrated that infants delivered before 39 weeks of gestation are at increased risk of adverse outcomes. Obviously, this argues against such practice and elective delivery before 39 weeks is strongly discouraged by major medical associations. Indeed, it's the number one recommendation of the American College of Obstetricians and Gynecologists in the Choosing Wisely campaign. In light of compelling evidence, the authors state that "the only remaining indications (for FLM testing) are restricted to those patients in whom accurate fetal dating is absent."

I could not agree more with these authors! If you want to hear more, you can watch a short video of me discussing this topic here.

FLM testing is being used with much less frequency than it has in the past and that's a good thing. Healthcare reform (in the US) is focusing its lens on quality healthcare practices and laboratory tests that don't contribute in a positive way should not be performed. I would argue that, with rare exceptions, FLM testing is not clinically useful. I have no doubt that if they were to vanish overnight the medical community would barely take notice. I look forward to that day.

The clinical utility of fetal lung maturity testing revisited 20 years later

There is a saying in science that every 10 or 20 years scientists "reinvent" things. This refers to observations someone made and published, but the findings were largely ignored for 10-20 years until a new scientist comes along and makes the same or similar observation and suddenly everyone takes notice.  It seems to me that is what is happening with fetal lung maturity testing.

As early as 1993 Wigton et al made the observation that in spite of documented fetal lung maturity (by L/S ratio or PG) major neonatal morbidity was observed in a population of 213 patients <37 weeks gestational age.  In 1997, Ghidini et al made a similar observation that the incidence of major neonatal complications among 153 preterm infants was high even in the presence of mature fetal lung tests.  I guess we didn't really pay attention to those papers since the Wigton paper has only been cited 16 times in 20 years and the Ghidini paper was cited 13 times in 16 years.

In March of 2011 David discussed a study by Bates et al, that had been published in 2010 (17 years after Wigton's paper)  which showed the very same thing in a larger population. They demonstrated that even after documented fetal lung maturity (L/S ratio or PG), infants born 36 to 39 weeks (n=459) were at higher risk of adverse outcomes than infants born at 39 to 40 weeks (n=13,339). Infants born before 39 weeks were, overall, at 1.6-fold greater risk of having something bad happen to them.  Things like elevated serum bilirubin, ventilator support, low blood glucose, admission to a neonatal intensive care unit, or even RDS. People are beginning to take notice because this paper has already been cited 20 times in just 2 years!

The Bates paper was followed in 2011 by a paper by Kamath et al Their study was not as large as Bates, but it broke down deliveries into late preterm (34 to 36 6/7 weeks; n=76) and early term (37 to 39 6/7 weeks; n=76) with documented fetal lung maturity as compared to infants greater than or equal to 39 weeks of gestation (n=262). These authors measured fetal lung maturity by TDx-FLM II, LBC, or PG. They again concluded that fetal lung maturity is insufficient to determine an infant's readiness for postnatal life.

So here we are in 2013, twenty years after Wigton's paper, and the largest study of preterm infants with mature lung indices has just been published by Fang et al. This study was very similar to the Bates study and compares infants born 36 to 38 6/7 weeks gestation with mature fetal lungs (as determined by LBC, L/S, or PG; n=1011) to infants 39-41 weeks of gestation (n=11,701).  They found that delivery prior to 39 weeks with documented fetal lung maturity was associated with an 8.4% composite neonatal morbidity rate as compared to 3.3% for deliveries at 39 weeks or greater. This is compared to 6.1% and 2.5% respectively for the Bates study. Fang observed that a large proportion (49%) of women in their study who were undergoing an amniocentesis to determine fetal lung maturity, between 36 and 38 weeks of gestation, had pregestational or gestational diabetes. Because this could be a confounding factor in their results, they excluded all diabetics and reanalyzed their data. They found that even in non-diabetic patients, significantly higher rates of neonatal morbidity persisted in the group that was delivered <39 weeks.

So what can we take away from these three "reinvented" papers? Certainly some pregnancy conditions require premature delivery. In these cases fetal lung maturity testing is irrelevant because the condition requires delivery regardless of lung maturity. In cases where premature delivery is not imminent, these studies show that gestational age itself has the strongest inverse correlation with morbidity.  Although fetal lung maturity testing may help to predict the absence of RDS, it does not mean that the infant will not have other complications due to immaturity. Essentially, delivery <39 weeks should be avoided regardless of fetal lung maturity testing. If lung maturity testing is performed, women should be counseled regarding the risk of neonatal morbidity even in the presence of a test results that indicates fetal lung maturity.

As David concluded in 2011, "perhaps it is time to send these tests away once and for all".

Diabetes, fetal lung maturity, and the lamellar body count

I've written a few posts on diabetes during pregnancy lately so thought I'd stick with it a bit longer. This time, however, the focus is on the effect that maternal diabetes has on fetal lung maturity.

It's generally believed that fetal lung maturity is delayed in women who have diabetes (gestational or otherwise). The reasons for this problem are not well understood, however high concentrations of glucose and insulin in the fetal blood have been postulated as a possible cause. Despite this, respiratory distress syndrome rarely occurs in term infants born to mothers with diabetes. Still, the issue of delayed fetal lung maturity and diabetes remains controversial.

I'll go on record for saying that there is no valid reason to perform amniocentesis solely to determine fetal lung maturity if maternal diabetes is well-controlled. However, if mom's diabetes is poorly controlled, then there may be value in performing fetal lung maturity tests. I'd like to emphasis the "may" in that last statement. As I've written before, it seems like all tests of fetal lung maturity should become obsolete. Realistically, however, they probably aren't going to go away any time soon.

Because the most widely used test of fetal lung maturity is about to disappear forever, many clinical laboratorians are planning to offer the lamellar body count test as a replacement. I've been asked several times what, if any, effect that maternal diabetes has on the results of this test.

Not surprisingly, that issue has been addressed only by a few published studies. In one study from 2002, a mature result from the LBC result was believable regardless of diabetes status (note, however, that this study did not include any neonates with RDS so a true assessment of the performance of the LBC could not be made). Another study from 2009 determined that the results of an LBC test were not influenced by maternal diabetes although it did suggest that the test was still useful in preterm pregnancies and in poorly controlled maternal diabetes.

What the take-home message? Granted, the data are limited but it's probably no more necessary to perform the LBC test in women with diabetes than it is in women without the disorder.

What the loss of a widely used fetal lung maturity test will mean to doctors and labs

The test most widely used to assess fetal lung maturity is going away.  Earlier this year I wrote about the different laboratory tests that doctors use to determine if a fetus' lungs were mature.  One of those tests, the surfactant-to-albumin ratio, is marketed and sold by Abbott Laboratories as the TDx FLM II test.  Last year, they announced that they were discontinuing the manufacture of that test.

The main reason this test will no longer be made is that the instrument that performs the test is several decades old.  The TDx line of instruments is aging and the technology it uses is becoming outdated.  The decision to stop supporting the TDx makes sense but it does leave labs that offer the FLM II assay in a bit of a quandary.  The Abbott test performs well and is rapid and simple to perform.  Doctors are familiar with it and know how to interpret the results.  The test is an excellent predictor of fetal lung maturity and the risk of respiratory distress syndrome as a function of both gestational age and TDx FLM II result is known.  Labs that currently offer the TDx FLM II test and that wish to continue offering a test for fetal lung maturity have to figure out what to do.  Doctors will also have to learn what the loss of this test will have on their clinical practice.

I recently published the results of a survey that provide some insight as to what might be expected. Here is a summary of those data:

  • Across the US, labs are performing fewer fetal lung maturity tests each year and the most widely used test is Abbott's TDx FLM II test.
  • Although 25% of physicians indicated that their use of fetal lung maturity tests is decreasing, 90% of them said that they could not provide their current level of care without access to a test of fetal lung maturity.
  • Without the TDx FLM II test, 65% of physicians would order the lecithin-to-sphingomyelin (L/S) ratio and 40% would order the lamellar body count (LBC).
  • Fewer than 20% of labs offer the L/S ratio and even fewer offer the LBC.
  • Laboratories could simply refer fetal lung maturity test requests to a lab that performs other lung maturity tests but doctors want results within 12 hours of ordering the test.  That expectation probably can't be realized if the test is not performed in the lab.

Of all the available tests that could potentially replace the TDx FLM II test, the LBC test is the most logical:

  • The instrumentation required to perform the test is an automated cell counter and these are widely available in clinical labs.
  • The test is performed rapidly and with high precision.
  • Several outcome-based studies have demonstrated the diagnostic utility of the LBC and it performs as well as the TDx FLM II test.

When I'm asked by laboratorians what they should do when the TDx FLM II test goes away I tell them that they should consider the LBC test.  Of all the other tests of fetal lung maturity, the LBC, for the reasons outlined above, is the most logical replacement test.  I also caution them that time is running out and that NOW is the time to communicate with their physicians about the need for the change and to get their input.

It's worth re-stating that perhaps, just maybe, we could consider stop performing fetal lung maturity tests altogether.  Time will tell.

    Fetal lung maturity tests. Are they truly necessary?

    When infants are born before 39 completed weeks of gestation, they are at increased risk of developing respiratory distress syndrome (RDS).  The risk of RDS increases as the gestational age of the infant at delivery decreases.  In other words, the more premature a baby is born, the more likely it is that it will have RDS.

    This is a primary reason why an elective delivery before 39 weeks of gestation should not take place unless the fetal lungs are shown to be mature using fetal lung maturity tests.  The logic behind that mandate should be obvious: infants that develop RDS can die and those that do survive often develop other serious complications like septicemia,  necrotizing enterocolitis, retinopathy, and developmental handicaps.

    Intuitively, it makes sense that if lab testing demonstrates mature fetal lungs then the risk of RDS, and its associated complications, would be low and elective delivery before 39 weeks could be permitted.  However, a recent study showed that even after documented fetal lung maturity, infants born before 39 weeks were at higher risk of adverse outcomes than infants born at 39 to 40 weeks.

    Infants born before 39 weeks were, overall, at 1.6-fold greater risk of having something bad happen to them.  Things like elevated serum bilirubin, ventilator support, low blood glucose, admission to a neonatal intensive care unit, or even RDS!

    Results like these definitely call into question the current convention that delivery before 39 weeks is okay if fetal lung maturity is confirmed by lab testing.  One has to even wonder why fetal lung maturity tests are even necessary.  Perhaps they aren't.

    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 declined considerably over the last 15 years.  Also, doctors are ordering fewer fetal lung maturity tests than they have in the past.  They probably aren't going to go away any time soon but all signs are pointing towards their demise.

    A growing body of evidence is telling us that 1) elective delivery of infants before 39 weeks of gestation should be avoided; 2) that's true even if their lungs are shown to be mature by lab testing; and 3) testing for fetal lung maturity is decreasing.

    Perhaps it is time to send these tests away once and for all.

    Tests of Fetal Lung Maturity

    This month the National Academy of Clinical Biochemistry (NACB) launched its blog, appropriately Slide1 called the NACBlog.  The first post is on the topic of fetal lung maturity tests and how the medical community might respond to the upcoming loss of a popular test of fetal lung maturity that is being discontinued by its manufacturer (disclaimer: I am the author of that post).

    Seems like a good reason to talk about FLM tests here!  So, how can a lab test evaluate a fetus' lungs?
    First the basics:

    1. The lungs are one of the last organs to mature in a fetus.  In order for them to work properly after birth the alveoli (i.e. air sacs) have to open up and stay open once the baby takes its first few breaths.  This is not as easy as it seems because the inside lining of the alveoli has a thin coating of water and the surface tension of this water promotes their collapse.  A collapsed alveoli doesn't work very well!  Fortunately, our lungs secrete chemicals called surfactants that lower the surface tension of the water that coats the alveoli thus preventing the alveoli from collapsing.
    2. Having enough of these surfactants in the lungs at birth is extremely important because that's when the baby has to make a transition from getting its oxygen from mom to getting it from the air.  If there's not enough surfactant then the alveoli may collapse and the baby may have a difficult time breathing.  When this does happen it's called respiratory distress syndrome of the newborn, or just RDS.
    3. Lung surfactants begin to be made around the 25th week of pregnancy but there's usually not enough of it present to prevent RDS until the 37th week.  That means babies born prematurely are at greater risk of developing RDS than those born at term.
    4. Because the fetus essentially "breathes" amniotic fluid in and out of its lungs, the amount of surfactant in the lungs can be determined by measuring surfactants in the amniotic fluid.  In the lab, there are a few ways we can do just that using a sample of amniotic fluid.

    Several fetal lung maturity tests have been developed since the 1970's but only a few are still in use today:

    • AmnioStat-FLM
      • This test looks for the presence of a lung surfactant called phosphatidylglycerol (usually just called PG).
      • This is an agglutination  test that uses antibodies to detect PG in amniotic fluid.  If PG is present then visible agglutinates (clumps of particles) can be seen and the fetal lungs are considered mature.
    • Lamellar Body Count
      • In certain cells of the lungs, surfactants are packaged into granules called lamellar bodies and secreted from the cells into the alveoli.  This test actually counts the number of lamellar bodies in amniotic fluid.
      • The higher the lamellar body count, the more likely it is that the fetal lungs are mature.
    • Lecithin/Sphingomyelin Ratio
      • This was the first test of fetal lung maturity ever developed and is more commonly known as the L/S ratio.  It's a measure of the ratio of two lung surfactants, lecithin and sphingomyeli, that's determined using a technique known as thin-layer chromatography.
      • Lecithin is the most important lung surfactant and provides the greatest surface tension-lowering properties of all the surfactants.  It increases dramatically in the last few weeks of pregnancy.  Sphingomyelin is a minor lung surfactant and that amount of it in the lungs stays about the same throughout pregnancy so it serves as a good baseline against which the increasing amount of lecithin can be compared.  A ratio that is 2.5 or greater is usually used to indicate lung maturity.
      • Many doctors consider this to be the "best" fetal lung maturity test but that is not true.
    • TDx FLM II
      • This test measures the ratio of surfactant to albumin and so is sometimes called the S/A ratio.
      • The test relies on a technique known as fluorescence polarization and is the most widely used fetal lung maturity test; unfortunately it will no longer be available to clinical labs at the end of this year because the manufacturer has decided to stop making it.
      • The effect that the loss of this test will have on patients, doctors, and labs remains to be seen!

    There a whole lot more to say about fetal lung maturity tests but those will have to wait for future posts.