The introduction and development of methods for assessing fetal lung maturity has
greatly improved the outcome of high-risk pregnancies, both emergent [
[1]
] and those in which the timing of delivery is scheduled [
[2]
]. Many tests of fetal lung maturity have been devised over the last 30 years; only
a handful of these tests are specifically designed to measure pulmonary surfactant—the
deficiency of which causes respiratory distress syndrome (RDS) [
[3]
]—and are unaffected by contaminants [
[4]
]. The authors focus on those tests that measure surfactant by chemical, physical,
or physicochemical approaches (Table 1). The sensitivity and specificity of these methods vary and should be evaluated carefully
because this information is critical to neonatal morbidity and mortality when used
as a discriminatory test of fetal lung maturity. The authors have deliberately avoided
those tests that do not specifically measure pulmonary surfactant (eg, creatinine,
lipid-staining cells, OD 650) and those novel tests that, although promising, do not
yet have a broad enough acceptance or enough prospective studies to justify their
recommendation (eg, refractive index-matched anomalous diffraction, nuclear magnetic
resonance spectroscopy).
Table 1Tests of fetal lung maturity
| Test | Reference | Normal value | Sensitivity; specificity | Affected by contaminants | Comments |
|---|---|---|---|---|---|
| L/S ratio |
[16]
|
≥2.0/1 | 1%–5%; 50% | Yes | “Gold standard” |
| Lung phospholipid profile |
[77]
|
PG > 1% of total phospholipid | <1%; <50% | Yes | Better specificity and sensitivity than L/S ratio |
| Saturated phosphatidylcholine |
[31]
|
500 mg/L | 1%; 82% | No | Best test for fetal lung maturity |
| Palmitic/stearic ratio | — | Varied results | Undefined | Yes | = L/S ratio |
| Fluorescence polarization |
94
,
95
,
101
|
50 mg/g of albumin | 96%; 88% | Yes | = L/S ratio |
| Surfactant protein A |
[69]
|
Qualitative | Limited experience | No | Unproven method |
| Shake test |
[43]
|
Grade I–V | 0.5%; 79% | Yes | Better than L/S ratio |
| Lamellar body count |
99
,
100
,
101
,
102
,
103
,
104
,
105
,
106
,
107
,
108
,
109
,
110
,
111
,
112
|
50,000/:L | Indeterminate | Yes | Better than L/S ratio |
Abbreviations: L/S, lecithin/sphingomyelin; PG, phosphatidylglycerol.
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