A 30-week preterm neonate develops respiratory distress and chest X-ray shows a 'ground glass' appearance with air bronchograms. Surfactant therapy is administered. Which lab measurement of amniotic fluid predicts fetal lung maturity most reliably?
- A Phosphatidylglycerol (PG) absent
- B Foam stability index < 0.47
- C Lecithin-to-sphingomyelin (L/S) ratio ≥ 2 ✓
- D Creatinine < 1.5 mg/dL
Correct answer: C. Lecithin-to-sphingomyelin (L/S) ratio ≥ 2
Explanation
The lecithin-to-sphingomyelin (L/S) ratio in amniotic fluid is the classic test for fetal lung maturity. L/S ≥ 2 indicates maturity (minimal risk of RDS), while L/S < 1.5 indicates high risk. Sphingomyelin remains constant throughout gestation as a reference; lecithin (DPPC) rises sharply after 35 weeks. Presence of phosphatidylglycerol (PG) further confirms maturity; its absence increases risk. The foam stability test and TDx-FLM assay are alternatives. Amniotic creatinine reflects fetal renal, not pulmonary maturity.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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