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Introduction ⬇

Fetal lung maturity (FLM) tests include amniotic fluid L/S ratio, PG agglutination test, surfactant-to-albumin (S/A) ratio, and lamellar body counts (LBC). FLM tests can be grouped into either biochemical tests (e.g., L/S ratio or Amniostat-FLM) or biophysical tests (e.g., LBC or S/A ratio). Lecithin and sphingomyelin, produced by lung tissue, have detergent-like action (pulmonary surfactant), which stabilizes the neonatal alveoli to prevent their collapse on expiration and consequent atelectasis. The amount of lecithin in amniotic fluid is less than the amount of sphingomyelin until 26 weeks of gestation; at 30–32 weeks of gestation, the two lipid values are about equal. At 35 weeks, lecithin level rises abruptly but sphingomyelin stays constant or decreases slightly. Saturated phosphatidylcholine, a subfraction of total lecithin, is a major surface-active component of lung surfactant.

The relationship between the phospholipids and the surface-active agents, L/S ratio, is used as an index of FLM. If early delivery is anticipated because of conditions such as diabetes, premature rupture of membranes, maternal hypertension, placental insufficiency, or erythroblastosis (Rh disease), the L/S ratio can be used to predict whether the fetal lung will function properly at birth. When early delivery is necessary for fetal viability, the result may be prematurity, pulmonary immaturity, or perinatal mortality. The L/S ratio should be determined on all repeat cesarean sections before delivery to ascertain when fetal lungs are functionally mature. Sphingomyelin exhibits surface-active properties in the lung but plays no role in the surfactant system except to be used as a convenient marker. The S/A ratio also indicates 
FLM.

Typically, screening for FLM is unnecessary if gestational age is >39 weeks. However, complications of maternal diabetes or Rh isoimmunization can restrict fetal lung development; therefore, assessment of FLM may be indicated. The AmnioStat-FLM-PG is a semiquantitative, immunologic, card agglutination rapid screening test that can determine the presence of PG, a component of pulmonary surfactant, to assess FLM. Another rapid screening test, the TDx FLM II system, uses fluorescence polarization technology to determine the S/A ratio. These rapid tests seem to be highly reliable in uncomplicated pregnancies.

LBC are measures of the phospholipid that represents the storage form of surfactant. The count is used to assess for FLM.

Procedure ⬆ ⬇

  1. Withdraw at least 3 mL of amniotic fluid, or collect from a free flow of fluid from the vagina in cases of ruptured membranes.

  2. Centrifuge the fluid and prepare for analysis and read the results in a reflectance densitometer. Calculate the L/S ratio.

Clinical Implications ⬆ ⬇

  1. A decreased L/S ratio (<1.5) is often associated with pulmonary immaturity and respiratory distress syndrome (RDS).

  2. An L/S ratio >2.0 signifies FLM. The occurrence of RDS is extremely unlikely.

  3. An L/S ratio between 1.5 and 1.9 indicates possible mild-to-moderate RDS (50% risk).

  4. Fetuses of women with type 1 diabetes (T1D) develop RDS at higher ratios. The L/S ratio should be >3.5 for these infants.

  5. A decreased S/A ratio (<40 mg/g) is consistent with immature fetal lungs.

  6. An S/A ratio of 40–55 mg/g is considered indeterminate of FLM status.

  7. LBC <15,000 particles/μL indicate fetal lung immaturity, whereas 15,000–60,000 particles/μL indicates transitional lung.

Clinical Alert

  1. If the L/S ratio is <1.5, it is preferable to delay induced delivery until the fetal lung becomes more mature.

  2. FLM appears to be regulated by hormonal factors, some stimulatory and others possibly inhi­bitory. For this reason, hormones such as betamethasone are given in two doses, administered 24 hours apart, if premature labor occurs.

  3. Under certain stressful conditions, premature fetal lung maturation may be seen. This accelerated fetal lung maturation is thought to be a protective mechanism for the preterm fetus should delivery actually occur.

    1. Premature rupture of the membranes. Prolonged rupture of the membranes (after 72 hours) has an acute negative effect on lung maturation

    2. Acute placental function

    3. Placental insufficiency

    4. Chronic abruptio placentae

    5. Renal hypertensive disease caused by degenerative forms of diabetes

    6. Cardiovascular hypertensive disease associated with drug abuse

    7. Severe pregnancy-induced hypertension

  4. Delayed fetal lung maturation may be seen in the following conditions; in these instances, a higher L/S ratio (>3.5) may be necessary to ensure adequate FLM:

    1. Infants born to patients with T1D

    2. Infants born to patients with nonhypertensive glomerulonephritis

    3. Hydrops fetalis

  5. A lung profile of amniotic fluid to evaluate lung maturity looks not only for lecithin but also for two other phospholipids—PG and phosphatidylinositol (PI). PI increases in the amniotic fluid after 26–30 weeks of gestation, peaks at 35–36 weeks, and then decreases gradually. PG appears after 35 weeks and continues to increase until term; measurements are classified as positive PG or negative PG. The lung profile is a useful adjunct in evaluating the L/S ratio. It appears that lung maturity can be confirmed in most pregnancies if PG is present (positive) in conjunction with an L/S ratio >2.0. PG may provide stability that makes the infant less susceptible to RDS when experiencing hypoglycemia, hypoxia, or hypothermia. The PG measurement is especially useful in borderline cases and in class A, B, and C of pregestational diabetes when pulmonary maturation is delayed.

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain the reason for testing and the meaning of results.

  2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Review test results; report and record findings. Modify the nursing care plan as needed.

  2. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆ ⬇

  1. High false-negative result rates

  2. Unpredictability or borderline values

  3. Unpredictability of contaminated blood specimens

  4. Occasional false-positive values associated with conditions such as Rh disease, 
diabetes, or severe birth asphyxia

Reference Values ⬆

Normal