An L:S ratio less than what indicates surfactant immaturity in fetal lung assessment?

• A. L:S > 3:1 indicates immaturity
• B. L:S < 2:1 indicates surfactant immaturity
• C. L:S ratio is not used in this assessment
• D. L:S = 2:1 indicates immaturity

Answer: (B)

Lecithin-to-sphingomyelin (L:S) ratio gauges surfactant production in the fetal lungs. Lecithin, mainly dipalmitoyl phosphatidylcholine, rises as the lungs mature, while sphingomyelin stays relatively constant. Because of this, the L:S ratio increases with advancing gestation. An L:S ratio below 2:1 indicates insufficient surfactant production, signaling pulmonary immaturity and a higher risk of respiratory distress syndrome after birth. When the ratio reaches 2:1 or higher, the lungs are generally considered mature enough to handle air breathing at birth, with many labs using 2.5:1 as a stricter cutoff or adding phosphatidylglycerol testing for confirmation. The other statements misstate the threshold or the use of the test: a higher ratio does not indicate immaturity, and the ratio is indeed used to assess maturity.