Lab Test: Alpha1 Fetoprotein (Amniotic Fluid) Level
- Measurement of alpha-fetoprotein in amniotic fluid for detection of open neural tube defect.
- 15 weeks' gestation: 16.3 mcg/mL (16.3 mg/L)
- 16 weeks' gestation: 14.5 mcg/mL (14.5 mg/L)
- 17 weeks' gestation: 13.4 mcg/mL (13.4 mg/L)
- 18 weeks' gestation: 12 mcg/mL (12 mg/L)
- 19 weeks' gestation: 10.7 mcg/mL (10.7 mg/L)
- 20 weeks' gestation: 8.1 mcg/mL (8.1 mg/L)
- Twin pregnancy: Values are 2 times higher
- Blacks: Values are 15% higher
- Type 2 diabetes mellitus: Values are 20% lower
- Prenatal diagnosis of suspected neural tube defect - Amniotic fluid alpha fetoprotein (AFP) values of 2 multiples of the median (MoM) or higher are seen in approximately 96% of pregnancies affected by open tube neural defects and 1% of unaffected pregnancies. It has a detection rate of 100% for detecting anecephaly.
- Elevated alpha-fetoprotein levels in amniotic fluid can also occur in ventral wall defects, congenital disorders, fetal bleeding fetal demise, and nephrotic syndrome.
- Level 2 ultrasound has become the preferred method for identifying congenital anomalies, except in cases where chromosomal analysis is indicated.
- Optimal time frame for testing is during the first part of the second trimester of pregnancy, between 14 and 16 weeks' gestation.
- Fetal blood contamination can cause falsely elevated AFP levels.
- Hemolysis of the specimen can alter results.
- Do not use a test tube with a rubber stopper to place collected sample. Instead use a sterile clean plastic container and after discarding the first 1-2 cc collected place at least 2.5 cc of amniotic fluid in it.
- Prior to doing the procedure, counseling should be done about the risk for miscarriage which is about 1:300-500 and chorioamnionitis in 1:1000.
- Evaluate the mother's blood pressure and the fetal heart rate.
- Follow instructions regarding emptying the bladder, which depend on gestational age. Before 20 weeks (but after 15 weeks) of gestation, the bladder may be kept full to support the uterus. After 20 weeks, the bladder may be emptied to minimize the chance of puncture.
- Localize the placenta by ultrasound examination before the study to permit selection of a site that will avoid placental puncture.
- Place the patient in the supine position.
- Note the following procedural steps:
- The skin overlying the chosen site (often determined by obstetric ultrasonography) is prepared and usually anesthetized locally.
- A needle with a stylet is inserted through the midabdominal wall and directed at an angle toward the middle of the uterine cavity.
- The stylet is then removed and a sterile plastic syringe attached.
- After 5 to 10 mL of amniotic fluid is withdrawn, the needle is removed.
- The specimen is placed in a light-resistant container to prevent breakdown of bilirubin.
- The site is covered with an adhesive bandage.
- If the amniotic fluid is bloody, the physician must determine whether the blood is maternal or fetal in origin. Kleihauer-Boetke stain will stain fetal cells pink. Meconium in the fluid is usually associated with a compromised fetus.
- Amniotic fluid volume is calculated by injecting a known concentration of solute (such as para-aminohippuric acid [PAH]) into the fluid to distribute throughout. Amniotic fluid is then withdrawn, and the PAH concentration is determined.
- This procedure is performed by a physician and takes approximately 20 to 30 minutes.
- Specimen is sent to a special chemistry laboratory for analysis.
- For women who have Rh-negative blood, administer RhoGAM because of the risk of immunization from the fetal blood.
- Assess the fetal heart rate after the test to detect any ill effects related to the procedure.
- If the patient felt dizzy or nauseated during the procedure, instruct her to lie on her left side for several minutes before leaving the examining room.
- Observe the puncture site for bleeding or other drainage.
- If the test is delayed, refrigerate sterile specimen up to a few hours.
- If longer delays occur, specimen should be frozen.
- Avoid exposure to heat, light and do not centrifuge.
- Explain the procedure to the patients and allow her to verbalize her concerns.
- Obtain an informed consent from the patient and her partner.
- Tell the patient that no food or fluid is restricted.
- Tell the patient that the discomfort associated with amniocentesis is usually described as a mild uterine cramping that occurs when the needle contacts the uterus. Some women may complain of a "pulling" sensation as the amniotic fluid is withdrawn.
- Inform the patient that the results of this study are usually not available for over 1 week.
- After the procedure, instruct the patient to call her physician if she has any amniotic fluid loss, bleeding, temperature elevation, abdominal pain or cramping, fetal hyperactivity, or unusual fetal lethargy.
The AFP assay is not done in isolation to identify neural tube defects. The result is interpreted in conjunction with confirmatory ultrasound. The test is performed on mothers whose pregnancies are considered to be high risk. Contraindications include patients with abruption placentae, patients with placenta previa, those with a history of premature labor, and those with an incompetent cervix. Complications may include miscarriage, fetal injury, leak of amniotic fluid, infection (amnionitis), abortion, premature labor, maternal hemorrhage with possible maternal Rh isoimmunization, amniotic fluid embolism, abruption placentae, and inadvertent damage to the bladder or intestines.
MESH Terms & Keywords