Which information regarding amniotic fluid is important for the nurse to understand?

Synonym/Acronym:
N/A

Rationale
To assist in identification of fetal gender, genetic disorders such as hemophilia and sickle cell anemia, chromosomal disorders such as Down syndrome, anatomical abnormalities such as spina bifida, alloimmune hemolytic disease of the newborn (HDN), and hereditary metabolic disorders such as cystic fibrosis. To assess for preterm infant fetal lung maturity to assist in evaluating for potential diagnosis of respiratory distress syndrome (RDS).

Patient Preparation
There are no food, fluid, activity, or medication restrictions unless by medical direction.

Normal Findings
Method: Macroscopic observation of fluid for color and appearance, immunochemiluminometric assay (ICMA) for α1-fetoprotein, electrophoresis for acetylcholinesterase, spectrophotometry for Cr, bilirubin, and chromatography for lecithin/sphingomyelin (L/S) ratio and phosphatidylglycerol, tissue culture for chromosome analysis, dipstick for leukocyte esterase, and automated cell counter for white blood cell count and lamellar bodies.

Critical Findings and Potential Interventions

• An L/S ratio less than 1.5:1 is predictive of RDS at the time of delivery.

Timely notification to the requesting health-care provider (HCP) of any critical findings and related symptoms is a role expectation of the professional nurse. A listing of these findings varies among facilities.

Infants known to be at risk for RDS can be treated with surfactant by intratracheal administration at birth.

Overview

Available rapid tests can be used to differentiate between amniotic fluid and other body fluids in a vaginal specimen collection. Nitrazine paper impregnated with an indicator dye will produce a color change indicative of vaginal pH. Normal vaginal pH is acidic (4.5–6), and the color of the paper will not change. Amniotic fluid has an alkaline pH (7.1–7.3), and the paper will turn blue. False-positive results occur in the presence of semen, blood, alkaline urine, vaginal infection, or antibiotic treatment. Amniotic fluid crystallization, or fern test, is the observation of a fern pattern when fluid air dries on a glass slide. The fern pattern is from the protein and sodium chloride content of the amniotic fluid. False-positive results occur in the presence of blood, urine, or cervical mucus. Both tests can produce false-negative results when a small amount of fluid is leaked. Result reliability is significantly diminished with the passage of time (greater than 24 hr). AmniSure is an immunoassay performed using a vaginal swab sample. This rapid test detects placental alpha microglobulin-1 (PAMG-1) protein, found in high concentrations in amniotic fluid. AmniSure does not have the high frequency of false-positive and false-negative results found with the pH and fern tests.

RDS is the most common problem encountered in the care of premature infants. RDS, also called hyaline membrane disease, results from a deficiency of phospholipid lung surfactants. The phospholipids in surfactant are produced by specialized alveolar cells and stored in granular lamellar bodies in the lung. In normally developed lungs, surfactant coats the surface of the alveoli. Surfactant reduces the surface tension of the alveolar wall during breathing. When there is an insufficient quantity of surfactant, the alveoli are unable to expand normally, and gas exchange is inhibited. Amniocentesis, a procedure by which fluid is removed from the amniotic sac, is used to assess fetal lung maturity.

Lecithin is the primary surfactant phospholipid, and it is a stabilizing factor for the alveoli. It is produced at a low but constant rate until the 35th wk of gestation, after which its production sharply increases. Sphingomyelin, another phospholipid component of surfactant, is also produced at a constant rate after the 26th wk of gestation. Before the 35th wk, the L/S ratio is usually less than 1.6:1. The ratio increases to 2 or greater when the rate of lecithin production increases after the 35th wk of gestation. Other phospholipids, such as phosphatidyl glycerol (PG) and phosphatidyl inositol (PI), increase over time in amniotic fluid as well. The presence of PG indicates that the fetus is within 2 to 6 wk of lung maturity (i.e., at full term). Simultaneous measurement of PG with the L/S ratio improves diagnostic accuracy. Production of phospholipid surfactant is delayed in mothers with diabetes. Therefore, caution must be used when interpreting the results obtained from a patient who is diabetic, and a higher ratio is expected to predict maturity.

HDN, also called erythroblastosis fetalis, is a condition that occurs after red blood cells (RBCs) from an Rh negative mother become sensitized by fetal RBCs from an Rh positive baby. Rh sensitization of the mother can result from a miscarriage, trauma such as a fall or blow to the abdominal area, after an invasive prenatal test (such as amniocentesis), or when the placenta detaches during birth. The mother’s immune system recognizes the baby’s RBCs as foreign and makes antibodies that cause the fetal RBCs to hemolyze. Bilirubin is a breakdown product of Hgb, the oxygen carrying protein in RBCs. Bilirubin measurements from amniotic fluid are used to screen for hemolysis in high risk situations, sometimes serial measurements are required to monitor elevated measurements or therapeutic interventions such as administration of RhIG, fetal transfusions, or a decision to deliver the baby.

Testing for common genetically transferred conditions can be performed on either or both prospective parents by blood tests, skin tests, or DNA testing. DNA testing can also be performed on the fetus, in utero, through the collection of fetal cells by amniocentesis or chorionic villus sampling. Genetics is the study and identification of genes, genetic mutations, and inheritance. For example, genetics provides some insight into the likelihood of inheriting a medical condition such as cystic fibrosis or of errors of amino acid metabolism. Knowledge of genetics assists in identifying those who may benefit from additional education, risk assessment, and counseling. Further information regarding inheritance of genes can be found in the study titled “Genetic Testing.” Counseling and written, informed consent are recommended and sometimes required before genetic testing.

Indications

• Assist in the diagnosis of (in utero) metabolic disorders, such as cystic fibrosis, or errors of lipid, carbohydrate, or amino acid metabolism.
• Assist in the evaluation of fetal lung maturity when preterm delivery is being considered.
• Detect infection secondary to ruptured membranes.
• Detect fetal ventral wall defects.
• Determine the optimal time for obstetric intervention in cases of threatened fetal survival caused by stresses related to maternal diabetes, toxemia, hemolytic diseases of the newborn, or postmaturity.
• Determine fetal gender when the mother is a known carrier of a sex-linked abnormal gene that could be transmitted to male offspring, such as hemophilia or Duchenne muscular dystrophy.
• Determine the presence of fetal distress in late-stage pregnancy.
• Evaluate fetus in families with a history of genetic disorders, such as Down syndrome, Tay-Sachs disease, chromosome or enzyme anomalies, or inherited hemoglobinopathies.
• Evaluate fetus in mothers of advanced maternal age (some of the aforementioned tests are routinely requested in mothers age 35 and older).
• Evaluate fetus in mothers with a history of miscarriage or stillbirth.
• Evaluate known or suspected hemolytic disease involving the fetus in an Rh-sensitized pregnancy, indicated by rising bilirubin levels, especially after the 30th wk of gestation.
• Evaluate suspected neural tube defects, such as spina bifida or myelomeningocele, as indicated by elevated α1-fetoprotein (see study titled “Maternal Markers” for information related to triple-marker testing).
• Identify fetuses at risk of developing RDS.

Interfering Factors

Contraindications

Women with a history of premature labor, incompetent cervix, or in the presence of placenta previa or abruptio placentae. There is some risk to having an amniocentesis performed, and the risk should be weighed against the need to obtain the desired diagnostic information. A small percentage (0.5%) of patients have experienced complications including premature rupture of membranes (PROM), premature labor, spontaneous abortion, and stillbirth.

Factors that may alter the results of the study

• Bilirubin may be falsely elevated if maternal Hgb or meconium is present in the sample; fetal acidosis may also lead to falsely elevated bilirubin levels.
• Bilirubin may be falsely decreased if the sample is exposed to light or if amniotic fluid volume is excessive.
• Maternal serum Cr should be measured simultaneously for comparison with amniotic fluid Cr for proper interpretation. Even in circumstances in which the maternal serum value is normal, the results of the amniotic fluid Cr may be misleading. A high fluid Cr value in the fetus of a mother who is diabetic may reflect the increased muscle mass of a larger fetus. If the fetus is big, the Cr may be high, and the fetus still may have immature kidneys.
• Contamination of the sample with blood or meconium or complications in pregnancy may yield inaccurate L/S ratios; fetal blood falsely elevates the L/S ratio.
• α1-Fetoprotein and acetylcholinesterase may be falsely elevated if the sample is contaminated with fetal blood.
• Karyotyping cannot be performed under the following conditions: (1) failure to promptly deliver samples for chromosomal analysis to the laboratory performing the test or (2) improper incubation of the sample, which causes cell death.

Potential Medical Diagnosis: Clinical Significance of Results

• Yellow, green, red, or brown fluid indicates the presence of bilirubin, blood (fetal or maternal), or meconium, which indicate fetal distress or death, hemolytic disease, or growth retardation.
• Elevated bilirubin levels indicate fetal hemolytic disease or intestinal obstruction. Measurement of bilirubin usually is not performed before 20 to 24 weeks’ gestation because no action can be taken before then. The severity of hemolytic disease is graded by optical density (OD) zones. A trend of increasing values with serial measurements may indicate the need for intrauterine transfusion or early delivery, depending on the fetal age. After 32 to 33 weeks’ gestation, early delivery is preferred over intrauterine transfusion because early delivery is more effective in providing the required care to the neonate.
• Cr concentration greater than 2 mg/dL (greater than 176.8 micromol/L) (SI Units = Conventional Units × 88.4)indicates fetal maturity (at 36–37 wk) if maternal Cr is also within the expected range. This value should be interpreted in conjunction with other parameters evaluated in amniotic fluid and especially with the L/S ratio because normal lung development depends on normal kidney development.
• An L/S ratio less than 2:1 and absence of phosphatidylglycerol at term indicate fetal lung immaturity and possible respiratory distress syndrome. Other conditions that decrease production of surfactants include advanced maternal age, multiple gestation, and polyhydramnios. Conditions that may increase production of surfactant include hypertension, intrauterine growth retardation, malnutrition, maternal diabetes, placenta previa, placental infarction, and premature rupture of the membranes. The expected L/S ratio for the fetus of a mother who is diabetic is higher (3.5:1).
• Lamellar bodies are specialized alveolar cells in which lung surfactant is stored. They are approximately the size of platelets. Their presence in sufficient quantities is an indicator of fetal lung maturity. Amniotic fluid lamellar body counts less than 15,000/microL are suggestive of immature lung development and predictive for increased risk of developing RDS; counts greater than 50,000/microL are predictive of mature lung development.
• Elevated AFP levels and presence of acetylcholinesterase may indicate a neural tube defect (see study titled “Maternal Markers”) related to leakage from the open spinal cord into the amniotic fluid. Elevation of AFP and/or acetylcholinesterase is also indicative of ventral wall defects. The presence of AFP and acetylcholinesterase in amniotic fluid is abnormal; however, the test is not a sensitive marker for neural tube defects; false-positive results can be caused by contamination of the sample with fetal blood, which normally contains measurable levels of AFP and acetylcholinesterase. Abnormal results can be confirmed by testing the amniotic fluid for the presence of fetal Hgb (Hgb F). If Hgb F is detected, then the specimen is likely contaminated with fetal blood and the results are unreliable.
• Abnormal karyotype indicates genetic abnormality (e.g., Tay-Sachs disease, intellectual disability, chromosome or enzyme anomalies, and inherited hemoglobinopathies). (See study titled “Chromosome Analysis, Blood.”)
• Elevated white blood cell count and positive leukocyte esterase are indicators of infection.

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