Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1049 page)

   Atomic absorption

   Target analyte: nonionized atomic lead

   Limit of quantitation: 1 μg/dL

   Anodic stripping

   Target analyte: oxidized lead

   Limit of quantitation: 1–2 μg/dL

   Requires sample pretreatment

   Inductively coupled plasma MS

   Target analyte: ions at mass/charge ratio of natural isotopes of Pb

   Limit of quantitation: <1 μg/dL

   Expensive technology

LECITHIN-TO-SPHINGOMYELIN (L:S) RATIO

   Definition

   The L:S ratio is based on the observation that there is outward flow of pulmonary secretions from the lungs into the AF, and this changes the phospholipid composition of AF, thereby enabling indirect assessment of fetal lung maturity. The concentrations of L and S in AF are approximately equal until 32–33 weeks of gestation, at which time the concentration of L begins to increase significantly, whereas the S concentration remains about the same. The measurement of S serves as a constant comparison for control of the relative increases in L because the volume of amniotic fluid cannot be accurately measured clinically. This technique involves TLC after organic solvent extraction. It is a difficult test to perform and interpret. The presence of blood or meconium can interfere with test interpretation. Empirically, the risk of respiratory distress syndrome (RDS) is exceedingly low when the L:S ratio is >2.0.