Fetal Lung Maturity

Aka: Fetal Lung Maturity, Lecithin to Sphingomyelin ratio, L/S ratio, Phosphatidylglycerol, Phosphatidylinositol, Total Surfactant Activity

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II. Indications: Assessment of Fetal Lung Maturity

  1. Gestational age of 34 weeks or more
  2. Assess risk of Hyaline Membrane Disease

III. Signs of adequate surfactant (requires Amniocentesis)

  1. Lecithin to Sphingomyelin ratio (L/S ratio) > 2:1
    1. Positive Predictive Value: 98% (if no comorbidity)
    2. Not accurate in Diabetes, fetal asphyxia, Rh Disease
  2. Phosphatidylglycerol (PG) present >3%
    1. More reliable than L:S ratio if maternal comorbidity
  3. Phosphatidylinositol (PI) present
  4. Desaturated Lecithin (DSL)
    1. Non-Diabetic Patient >1500
    2. Gestational Diabetes Mellitus >2000
    3. Most sensitive of all lung maturity tests
  5. Total Surfactant Activity tests
    1. Fetal Lung Maturity Test (TDX-FLM, Abbott Labs) >70%
    2. Foam Stability Index (Beckman Inst.) >47

IV. Bedside Tests

  1. Tap Test
    1. Mix amniotic fluid with indicators
      1. Start with 1 ml amniotic fluid
      2. Add one drop of 6N Hydrochloric acid
      3. Add 1.5 ml Diethyl ether
    2. Tap tube several times
      1. Forms several hundred bubbles in ether layer
    3. Interpretation
      1. Mature if <6 bubbles in ether layer at 10 minutes
        1. Surfactant expedites bubbles breaking down
  2. Turbidity Test
    1. Hold amniotic fluid tube over newsprint
    2. Mature if fluid prevents reading newsprint
      1. Amniotic fluid turbidity increases with surfactant
      2. Positive Predictive Value: 97%

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