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- Fetal Lung Maturity (FLM)
Results of this test are for investigational purposes only. The performance characteristics of this assay have been determined by LabCorp. Results should not be used as a diagnostic procedure without confirmation of the diagnosis by another medically established diagnostic product or procedure.
• Immature lung: <8 x 103/μL
• Transitional lung: 8−32 x 103/μL
• Mature lung: >32 x 103/μL
Respiratory distress syndrome (RDS) is a serious condition with significant mortality that affects 10.5% of infants born at 34 weeks and 0.3% of infants born at 38 weeks of gestation.1 It is caused by insufficient production of pulmonary surfactant by immature fetal lungs. Recent advancements in maternal and neonatal care have helped decrease mortality from RDS in the United States from 12% in 1979 to 0.2% in 2003.2 Laboratory testing for fetal lung maturity (FLM) includes surfactant:albumin ratio by fluorescent polarization (FPOL), lamellar body counts (LBC), phosphatidylglycerol (PG), and lecithin:sphingomyelin ratio (L:S ratio) assays.2 The current "gold standard" is the evaluation of phospholipids (L:S ratio and PG) in amniotic fluid; however, these tests are difficult to perform and time-consuming.2,3
Most laboratories use the FPOL assay, which has a high predictive value for maturity, as the first-line rapid screening test. If results are immature or transitional, the sequential testing approach is recommended until a mature result is obtained or all test options are exhausted.4,5 The sole manufacturer of the FPOL test (Abbott Laboratories) has discontinued production of its TDx FLM ll™ assay. Consequently, LabCorp has discontinued FPOL testing. The lamellar body counts test is reported to have similar clinical performance as the FPOL assay with superb analytical characteristics.4,6
Lamellar bodies are composed from a surfactant packaged into intracellular storage granules. Fetal breathing movements in utero release lamellar bodies into amniotic fluid at 28 to 32 weeks of gestation, and their concentration increases exponentially as gestation continues.2 Thus, LBC measurement assists estimation of FLM during weeks 32 to 36 of gestation. By week 37 of gestation, the risk of RDS is so low that the laboratory assessment of FLM is rarely indicated.2 Due to the similar size of lamellar bodies and platelets, automated hematology analyzers can quantify LBC in amniotic fluid.2
Studies report that clinical performance of LBC in noncentrifuged amniotic fluid demonstrates sensitivity in the range of 92% to 100%, specificity in the range of 37% to 98%, positive predictive value (for immaturity) in the range of 24% to 77%, and negative predictive value (for maturity in the range of 98% to 100%.2 Very high maturity predictive value makes the LBC assay a test of choice for a rapid FLM screening.4-6 It is also important to note that different laboratories are using different analyzers, and clinical decision limits may vary depending on the platelet counting method in use.3,5
Amniotic fluid is obtained by needle aspiration into the amniotic sac (amniocentesis). Do not centrifuge. Do not freeze. Do not vortex.
Causes for Rejection
|Order Code||Order Code Name||Order Loinc||Result Code||Result Code Name||UofM||Result LOINC|
|005038||Lamellar Body Counts (LBC)||005039||Color, Amniotic||38386-9|
|005038||Lamellar Body Counts (LBC)||005040||Appearance, Amniotic||1887-9|
|005038||Lamellar Body Counts (LBC)||005042||LBC, Amniotic||x10E3/uL||19114-8|