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Predicting fetal lung maturity and assessing the risk of developing neonatal respiratory distress syndrome, when performed during 32 to 39 weeks gestation
Fetal lung maturity testing is used to determine the risk for developing respiratory distress syndrome (RDS) in infants born prematurely (32-39 weeks). The risk for developing RDS is inversely related to gestational age and is the most common cause of respiratory failure in neonates. RDS is associated with preterm birth due to insufficient production of pulmonary surfactant. Pulmonary surfactant is synthesized by type II pneumocytes. Surfactant consists of 90% phospholipids (primarily phosphatidylcholine and phosphatidylglycerol) and 10% proteins (surfactant proteins [SP]-A, SP-B, SP-C). Surfactant is packaged into lamellar bodies and is excreted into the alveolar space where it unravels and forms a monolayer on alveolar surfaces. Lamellar bodies can also pass into the amniotic cavity and, hence, are found in amniotic fluid. The surfactant functions to reduce the surface tension in the alveoli, preventing atelectasis. When surfactant is deficient, the small alveoli collapse and the large alveoli become overinflated and stiff, which has been associated with increased risk of developing respiratory distress. The status of fetal lung maturity is reflected in the concentration of surfactant in the form of phospholipids (see also FLP / Fetal Lung Profile, Amniotic Fluid) and lamellar bodies present in amniotic fluid. Lamellar bodies are similar in size to platelets and can be quantified on a hematology analyzer utilizing the platelet channel and used to estimate fetal lung maturity.
FLP / Fetal Lung Profile, Amniotic Fluid and LBC / Lamellar Body Count, Amniotic Fluid are individually orderable. A reflexive testing algorithm (LBCR / Lamellar Body Count Reflex, Amniotic Fluid) is available to reflex indeterminate lamellar body count results for FLP / Fetal Lung Profile, Amniotic Fluid (lecithin/sphingomyelin [L/S] ratio and phosphatidylglycerol [PG] testing).
Cutoffs are based on consensus protocol (Neerhof M, Dohnal JC, Ashwood ER, et al: Lamellar body counts: a consensus on protocol. Obstet Gynecol 2001;97:318-320)
Amniotic fluid lamellar body count (LBC) >50,000/mcL is predictive of fetal lung maturity.
Amniotic fluid LBC <15,000/mcL is suggestive of fetal lung immaturity and increased risk of neonatal respiratory distress syndrome (RDS).
The main value of fetal lung maturity testing is predicting the absence of RDS. An immature test result for fetal lung maturity is less reliable in predicting the presence of RDS.(1)
Surfactant secretion into the amniotic fluid is minimal prior to 32-weeks gestation.
Fetal lung maturity testing is not indicated beyond week 39.
Specimens should not be frozen or centrifuged. Freezing and centrifuging the amniotic fluid falsely decreases the lamellar body count. Amniotic specimens should be blood and meconium free.
1. Fetal Lung Maturity. ACOG Practice bulletin. Obstet Gynecol September 2008;112(3):717-726
2. Eby C, Lu J, Gronowski AM: Lamellar Body Counts Performed on Automated Hematology Analyzers to Assess Fetal Lung Maturity. Lab Med 2008;39(7):15
3. Haymond S, Luzzi V, Parvin C, Gronowski A: A Direct Comparison Between Lamellar Body Counts and Fluorescent Polarization Methods for Predicting Respiratory Distress Syndrome. Am J Clin Pathol 2006;126:894-899
4. Szallasi A, Gronowski A, Eby C: Lamellar Body Count in Amniotic Fluid: A Comparative Study of Four Different Hematology Analyzers. Clin Chem 2003;49(6):994-997
5. Grenache DG, Gronowski AM: Fetal lung maturity. Clin Biochem 2006;39:1-10
6. Neerhof M, Dohnal JC, Ashwood ER, et al: Lamellar body counts: a consensus on protocol. Obstet Gynecol 2001;97:318-320