Tests for Fetal Lung Maturity
The primary laboratory tests historically used to assess fetal lung maturity include lecithin/sphingomyelin (L/S) ratio, phosphatidylglycerol (PG), lamellar body count (LBC), and surfactant/albumin ratio, though current clinical practice has largely moved away from routine fetal lung maturity testing in favor of gestational age-based decision making.
Historical Laboratory Tests
The following tests were developed to measure surfactant concentrations in amniotic fluid obtained via amniocentesis:
Biochemical Tests
- Lecithin/Sphingomyelin (L/S) Ratio: Measures the ratio of these two phospholipids; a ratio ≥2.0 traditionally indicated lung maturity 1, 2, 3
- Phosphatidylglycerol (PG): Presence indicates lung maturity; absence suggests immaturity 1, 2, 3
- Surfactant/Albumin Ratio (TDx FLM II): Was the most frequently used test but is no longer available as of December 2011 1, 4
- Disaturated Lecithin: Measures the active component of surfactant 5
Physical Tests
- Lamellar Body Count (LBC): Counts surfactant-containing particles; values ≥50,000/μL suggest maturity 1, 4, 3
- Shake Test (Foam Stability Index): Simple bedside test measuring surfactant's ability to generate stable foam 2, 6
- Tap Test: Bedside alternative with high sensitivity (96-100%) and specificity (100% at 5-10 minutes) 6
Advanced Techniques
- Fluorescence Polarization: Rapid automated test that was widely used 2
- Quantitative Ultrasound (quantusFLM): Non-invasive technique analyzing fetal lung texture via ultrasound and machine learning 7
Current Clinical Reality
Fetal lung maturity testing has essentially reached the end of clinical utility 5, 8. Here's why:
The Paradigm Shift
The 2019 ACOG guidelines state that FLM testing no longer has clinical utility 5. This represents a fundamental change because:
- Gestational age is the strongest predictor of neonatal respiratory outcomes, not biochemical test results 8
- Demonstration of mature FLM test results does not improve neonatal outcomes 8
- Test volumes have declined precipitously—many reference laboratories discontinued FLM testing between 2016-2019 due to extremely low demand 5
- The shift toward managed care and evidence-based practice revealed these tests were not cost-effective 3
When Corticosteroids Replace Testing
Current guidelines emphasize antenatal corticosteroid administration rather than FLM testing:
- Administer corticosteroids between 24+0 and 34+0 weeks when preterm delivery is anticipated 9
- May be given up to 38+0 weeks for elective cesarean section 9
- For vasa praevia specifically, give corticosteroids from 32 weeks due to increased preterm delivery risk 10
- Multiple courses are not recommended 9
Practical Algorithm for Modern Practice
If considering delivery before 39 weeks:
- Use gestational age as primary determinant, not FLM testing
- Administer corticosteroids if delivery anticipated between 24-34 weeks
- Avoid amniocentesis for FLM testing—the invasive procedure carries miscarriage risk without improving outcomes 7
- Base timing decisions on maternal/fetal indications, not biochemical maturity markers
Common Pitfall to Avoid
Do not order FLM testing to justify early delivery decisions. The tests were designed to predict respiratory distress syndrome, but their positive predictive value for lung maturity (56-67% for most tests) is poor compared to simply using gestational age 6. The negative predictive value (ruling out immaturity) was better, but this clinical question is now addressed by administering corticosteroids rather than testing.
Clinical laboratory directors should meet with obstetrics providers to discontinue any remaining FLM testing services 5, as these tests represent outdated practice that does not align with current evidence-based guidelines prioritizing gestational age and prophylactic corticosteroid administration.