Tests for determining fetal lung maturity

Test	Positive discriminating value	Predictive value for pulmonary maturity when test is mature*	Predictive value for pulmonary immaturity when test is immature*	Pros and cons
Lamellar body count	30,000 to 40,000^¶	97 to 98%	29 to 35%	Results affected by blood but not by meconium. Cannot use a vaginal pooled sample.
Phosphatidylglycerol	Present (typically greater than 2% of total phospholipids when performed by thin-layer chromatography; at least 0.5 mg/L when performed by slide agglutination)	95 to 100%	23 to 53%	Time-consuming, complex test if performed using thin-layer chromatography, but a rapid and simple immunological method (slide agglutination) that uses antisera to phosphatidylglycerol is available. Not affected by blood or meconium. Bacteria can give a false positive in vaginal pool samples.
Lecithin/sphingomyelin ratio	Greater than or equal to 2	95 to 100%	33 to 50%	Time-consuming, complex test with large laboratory variation. Results affected by blood and meconium. Avoid use of a vaginal pooled sample.

* The predictive value for each test is influenced by the prevalence of respiratory distress syndrome in the population; thus, it varies with gestational age.
¶ ≥50,000 is the current threshold for maturity (refer to text).

Adapted from: ACOG Practice Bulletin No. 97: Fetal Lung Maturity. Obstet Gynecol 2008; 112:717.

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Tests for determining fetal lung maturity