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At a Glance
Respiratory distress syndrome (RDS), also known as hyaline membrane disease (HMD), is most frequently caused by insufficient surfactant production, resulting in structural lung immaturity in newborns. The risk of RDS is highest in premature births (>60% at 29 weeks of gestation, 20% at 34 weeks of gestation, and <5% at 37 weeks of gestation or later). Therefore, delivery before term increases the risk of RDS in an infant.
What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?
Lamellar body counts (LBCs) on amniotic fluid is the best test to assess fetal lung maturity. It has a high predictive value for a mature lung result and a rapid turnaround time. The test is performed on freshly collected amniotic fluid. Because of the similarity in size between platelets and lamellar bodies, lamellar bodies can be counted using the platelet channel of a hematology analyzer. An LBC greater than 50,000/µL is indicative of fetal lung maturity (FLM). Using this cutoff, sensitivity for RDS is 92-100% and specificity is 40-90%. (Table 1)
Table 1.
| LBC | Interpretation |
|---|---|
| ≥ 50,000/µL | Mature |
Alternative tests include the Lecithin/Sphyngomyelin (L/S) ratio. However, this testing is only performed by a limited number of laboratories, has a longer turnaround time, and does not outperform the LBC test. Phosphatidylglycerol (PG) measurement by thin layer chromatography (TLC) along with the L/S ratio is available in some laboratories. In addition, PG can also be measured using the AmnioStat-FLM test, which has a shorter turnaround time. A result greater than or equal to 2.0 on L:S ratio indicates maturity; presence of PG in the TLC method indicates maturity as in the AmnioStat-FLM.
Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications – OTC drugs or Herbals – that might affect the lab results?
LBC can be affected by the presence of blood and meconium in the sample. Whole blood contamination with a red blood cell count greater than 31,000/µL can falsely decrease LBC results, which is thought to be due to fibrin matrix trapping of lamellar bodies. Meconium contamination can falsely increase LBC results because of contaminants that can be detected by the platelet channel. In addition, it is important not to freeze the amniotic fluid samples before testing, as freezing and thawing fragments lamellar bodies into smaller particles that are not detected by the hematology counter, leading to falsely low results.
What Lab Results Are Absolutely Confirmatory?
There are no prenatal tests to tell with certainty if a fetus will develop RDS after birth. Development of RDS is affected by how long after FLM testing the fetus is delivered and whether the mother has been given steroids to hasten fetal lung development.
The American College of Obstetricians and Gynecologists (ACOG) does not recommend testing for fetal lung maturity if delivery is mandated by fetal or maternal indications. In addition, testing should be performed only if delivery is desired but can be safely delayed.
If waiting for term birth is not an option, antenatal corticosteroid treatment of the mother can accelerate fetal lung maturation. Current ACOG guidelines recommend a single course of steroids for women at risk of preterm birth between 24 and 34 weeks of gestation. Recent work suggests that the same treatment for pregnancies over 34 gestation weeks yields similar increases in fetal lung maturity.
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