Amniocentesis (Amniotic Fluid Analysis)
Test Description
Amniocentesis is an invasive diagnostic procedure which involves the transabdominal
needle aspiration of amniotic fluid. A 10 to 20 mL sample of the amniotic fluid
is withdrawn for analysis. This test is useful in detecting chromosomal abnormalities
such as Down syndrome and neural tube defects such as spina bifida, in determining
fetal maturity, and in detecting hemolytic disease of the newborn due to Rh
incompatibility. It can also be used for determining gender, although the test is not
done with this purpose in mind unless there is question of an X-linked chromosomal
abnormality. The timing of the procedure is based on the reason the procedure
is being done. If it is to determine genetic abnormalities, it is usually performed
after the fifteenth week of pregnancy, when the two layers of fetal membranes have
fused sufficiently to allow safe withdrawal of the amniotic fluid sample. To determine
fetal maturity, the procedure is more likely to be performed after the thirty
sixth week of gestation.
The amniotic fluid is analyzed for the following: acetylcholinesterase, alphafetoprotein,
bacteria, bilirubin, chromosomal karyotype, color, creatinine, glucose,
lecithin-to-sphingomyelin (L/S) ratio, meconium, and phosphatidylglycerol (PG).
Hemolytic disease of the newborn is indicated by high levels of bilirubin in the fluid.
Meconium staining indicates possible fetal distress. Fetal pulmonary immaturity is
demonstrated by a low L/S ratio and the absence of PG. A decreased creatinine level
may also indicate fetal immaturity. Neural tube defects are suspected with
increased levels of alpha-fetoprotein and acetylcholinesterase
THE EVIDENCE FOR PRACTICE
Use of amniocentesis is recommended by the American College of Obstetricians and
Gynecologists as follows
• Women pregnant with a single fetus who will be 35 years or older at delivery should be
offered prenatal diagnosis for fetal aneuploidy.
• Counseling for amniocentesis in a twin pregnancy in women age 33 years is indicated
because the midtrimester risk of fetal Down syndrome is approximately the same as for
that of a single pregnancy at age 35 years.
• Patients with a high risk of fetal aneuploidy include women with a previous pregnancy
complicated by a chromosomal abnormality, a major fetal structural defect identified by
ultrasonography, either parent with a chromosome translocation, and carriers of a chromosomal
abnormality.
Normal Values
Acetylcholinesterase: Absent
Alpha-fetoprotein: Varies with gestational age (peaks at 13–14 weeks)
Bacteria: Absent
Bilirubin: Absent at term
Chromosomes: Normal karyotype
Color: Colorless or light straw
Creatinine: >2 mg/100 mL when mature
Glucose: <45 mg/100 mL
L/S ratio: >2 indicates pulmonary maturity
Meconium: Absent
Phosphatidylglycerol: Present with pulmonary maturity
Possible Meanings of Abnormal Values
Chromosomal abnormalities (e.g., Down syndrome)
Fetal distress
Genetic aberrations (e.g., galactosemia)
Hereditary metabolic disorders (e.g., cystic fibrosis)
Neural tube defects (e.g., spina bifida)
Pulmonary immaturity
Rh isoimmunization
Sex-linked disorders (e.g., hemophilia)
Sickle cell anemia
Thalassemia
Contributing Factors to Abnormal Values
• Alpha-fetoprotein and acetylcholinesterase may be falsely elevated if the sample is
contaminated with fetal blood.
• Bilirubin may be falsely elevated if the sample is contaminated with maternal hemoglobin
or if meconium is present in the sample.
• Bilirubin may be falsely low if the sample is exposed to light.
Interventions/Implications
Pretest
• Explain to the patient the purpose of the test and procedure to be followed.
• No fasting is required prior to the test.
• Obtain a signed informed consent.
• If an ultrasound is to be performed just prior to the amniocentesis, the patient needs to
be well-hydrated and have a full bladder. The bladder must then be emptied prior to the
amniocentesis to avoid accidental puncture.
Procedure
• The patient is assisted to a supine position.
• The skin of the lower abdomen is cleansed with an antiseptic solution and draped.
• The skin overlying the chosen site is anesthetized with 1% lidocaine.
• A 20-gauge spinal needle is inserted into the amniotic cavity and the stylet is withdrawn.
A 10-mL syringe is then attached to the needle.
• The fluid sample is aspirated and placed in an amber or foil-covered test tube. This will
protect the sample from light and avoid breakdown of bilirubin in the sample.
• The needle is withdrawn, and a dressing is applied over the site.
• Gloves are worn throughout the procedure.
Post test
• Monitor fetal heart rate and maternal vital signs every 15 minutes until stable.
• If the patient complains of nausea or feeling faint, assist her to lie on her left side. This
will relieve uterine pressure on the vena cava.
• Observe the puncture site for drainage.
• Instruct the patient to notify the health-care provider immediately if any of the following
occur: abdominal pain, cramping, chills, fever, vaginal bleeding, fetal hyperactivity, or
unusual fetal lethargy.
• Protect the specimen from light, label it, and transport it to the laboratory immediately.
• Inform the patient that results may not be available for up to 3 weeks.
• Report abnormal findings to the primary care provider.
Clinical Alerts
• Possible complications include: amniotic fluid embolism, fetal injury, hemorrhage,
infection, premature labor, Rh sensitization due to fetal bleeding into
maternal circulation, and spontaneous abortion.
• It is possible for some fetal blood cells to enter the mother’s bloodstream during
the procedure. If the mother’s blood is Rh negative and the fetus’ is Rh positive,
the mother may produce antibodies against the fetus. To avoid this, RhoGAM
should be given to the mother.
CONTRAINDICATIONS!
• Patients with abruptio placentae, incompetent cervix, placenta previa
• Patients with a history of premature labor
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