Amniotic Fluid Alpha1-Fetoprotein (AFP)
AFP is synthesized by the embryonic liver and is the major protein (glycoprotein) found in fetal serum. It resembles albumin in molecular weight, amino acid sequence, and immunologic characteristics. However, it is not normally detectable after birth. Ordinarily, high levels of fetoproteins are found in the developing fetus, and low levels exist in maternal serum and amniotic fluid.
The amniotic fluid AFP test is used to diagnose fetal neural tube defects (malformations of the central nervous system); fetoprotein leaks into the amniotic fluid during such pregnancies. The causes of neural tube defects are not known; however, a genetic component is assumed because an increased risk for recurrence exists. Neural tube defects usually exhibit polygenic traits (added effects of genes at multiple loci). In cases of anencephaly and open spina bifida, both MSAFP and amniotic fluid AFP concentrations are abnormal by the 18th week of gestation.
In addition, AFP measurements have been used as indicators of fetal distress; in such cases, both amniotic fluid AFP and MSAFP may be increased. However, final confirmation must come from further studies.
Increased amniotic AFP levels are associated with:
Neural tube defects such as anencephaly (100% reliable), encephalocele, spina bifida, and myelomeningocele (90% reliable)
Congenital nephrosis
Omphalocele
Turner syndrome with cystic hydromas
Gastrointestinal tract obstruction
Missed miscarriage
Fetal distress
Imminent or actual fetal death
Severe Rh immunization
Esophageal or duodenal atresia
Fetal liver necrosis secondary to herpesvirus infection
Sacrococcygeal teratoma
Spontaneous miscarriage
Trisomy 13
Urinary obstruction (e.g., fetal bladder neck obstruction with hydronephrosis)
Cystic fibrosis
Pretest Patient Care
Explain the test purpose and the meaning of positive and negative test results.
Provide for genetic counseling.
Follow guidelines in Chapter 1 for safe, effective, informed pretest care.
Posttest Patient Care
Review test results; report and record findings. Modify the nursing care plan as needed.
Counsel the patient and monitor appropriately.
Follow guidelines in Chapter 1 for safe, effective, informed posttest care.
Fetal blood contamination causes increased AFP.
Increased AFP is associated with multiple pregnancies.
False-positive (0.1%–0.2%) results may be associated with fetal death, twins, or genetic anomalies, but sometimes, no explanation can be given for the results.
Clinical Alert
Any parents who have already produced a child with a neural tube defect should be offered antenatal studies in anticipation of future pregnancies. If one parent has spina bifida, the pregnancy should be closely monitored