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Lab Test: Alpha 1-Fetoprotein (AFP; Blood)

    Lab Test

    Alpha 1-Fetoprotein (AFP) (Blood)

    Description
    • Alpha 1-Fetoprotein (AFP) is an oncofetal protein produced by the yolk sac and liver of the developing fetus that increases in concentration throughout pregnancy. 
    • It is commonly used a as a screening or marker to identify any increased risk for birth defects, with neural tube defects (including spina bifida) being the most common. 
    • However, it can also be used as a tumor marker to identify cancers.
    Reference Range
    • Adult:  < 40 ng/mL or < 40 mcg/L (SI units)
    • Child younger than 1 year:  < 30 ng/mL
      • Ranges are stratified by weeks of gestation and vary among laboratories.
    Indications & Uses
    • Congenital Anomalies:  Since AFP is an oncofetal protein that is normally produced by the fetal liver and yolk sac, it is a dominant fetal serum protein in the 1st trimester and declines rapidly during the first year of life.  It can be detected in the amniotic fluid or the mother's blood after 10 weeks' gestation and peaks between 16 and 18 weeks.  If elevated AFP levels are identified, the additional evaluation is needed.
    • Tumor evaluation:  Approximately 90% of patients with hepatomas will have elevated levels (usually > 500 ng/mL) and the higher the AFP level, the worse the tumor burden.  In fact, a decrease in AFP can be seen in patients responding to chemotherapy.
    • Testing methods for AFP quantification include radioimmunoassay or enzyme-linked immunosorbent assay (ELISA) with a commercially available kit.
    Clinical Application
    • Increased maternal serum levels:
      • Neural tube defects (e.g., anencephaly, encephalocele, spina bifida, myelomeningocele), abdominal wall defects (e.g., gastroschisis, omphalocele): 
        • An open body wall defect will cause fetal serum AFP to leak out into the amniotic fluid where it is picked up by the maternal serum.  While a positive maternal serum AFP result is not diagnostic for NTD it warrants further evaluation to include acetylcholinesterase measurement of amniotic fluid in conjunction with ultrasound.  AFP is known to have an 80-85% detection rate for presence of spina bifida and 90-95% for anencephaly with a 1.5-2.5% false positive rate.
      • Multiple-fetus pregnancy
      • Threatened abortion, fetal distress or death
      • Congenital anomalies:
        • If AFP is used by itself it only has a 20-25% detection rate for the presence of Down syndrome.
    • Increased nonmaternal serum levels:
      • Primary hepatocellular cancer (hepatoma)
      • Germ cell or yolk sac cancer of the ovary
      • Embryonal cell or germ cell tumor of the testes
      • Other cancers (e.g., stomach, colon, lung, breast, lymphoma).
    • Decreased maternal levels:
      • Trisomy 21 (Down Syndrome):  A low AFP and low unconjungated estriol (uE3) levels along with elevated levels of human chorionic gonadotrophin (hCG) identify women at sufficient risk for Down syndrome who warrant further testing.  Correction for maternal insulin dependent diabetes mellitus should be made for AFP. 
    • Trisomy 18 (Edward syndrome):  Decreased levels of hCG, AFP, and uE3 identify women at sufficient risk and warrant further testing.
    Related Tests
    • Maternal screening test
    • Amniocentesis
    • Pelvic ultrasonography - nuchal translucency is an accurate indicator of trisomy chromosomal abnormalities
    Drug-Lab Interactions
    • Fetal blood contamination, which may occur during amniocentesis, can cause increased AFP levels.
    • Multiple pregnancies can cause increased levels.
    • Recent administration of radioisotopes can affect values because results are determined by radioimmunoassay.
    Test Tube Needed
    • Draw 5 mL blood in red top or serum separator tube
    Procedure
    • Collect a venous blood sample between 16-18 weeks gestation. 
    • Apply pressure or a pressure dressing to the venipuncture site and assess the site for bleeding.
    • Include the gestational age on the laboratory slip.
    Storage and Handling
    • Centrifuge within 2 hours of collection and transfer serum into a clean plastic vial
    • Serum may be stored at 4°C to 8°C for days or at −20°C for years.
    What To Tell Patient Before & After
    • Explain the procedure to the patient and tell them that no food or fluid restriction is required.
    References
    • Canick JA et al. Clin Lab Med 2003;23(2):385-94.
    • Rose NC et al. West J Med 1993;159(3):312-7.

MESH Terms & Keywords

  • Alpha 1-Fetoprotein, AFP Neural Tube Defect, Blood AFP Test