Alpha-Fetoprotein (AFP) test is used as a screening marker indicating increased risk for birth defects, such as fetal body wall defects, neural tube defects, and chromosomal abnormalities. It can also be used as a tumor marker to identify cancers.
Alpha-Fetoprotein is an oncofetal protein normally produced by the fetal liver and yolk sac. It is the dominant fetal serum protein in the first trimester of life and diminishes to very low levels by the age of 1 year. Normally it is found in very low levels in the adult.
Alpha-Fetoprotein is an effective screening serum marker for fetal body wall defects. The most notable of these is neural tube defects, which can vary from a small myelomeningocele to anencephaly. If a fetus has an open body wall defect, fetal serum Alpha-Fetoprotein leaks out into the amniotic fluid and is picked up by the maternal serum. Normally Alpha-Fetoprotein from fetal sources can be detected in the amniotic fluid or the mother’s blood after 10 weeks’ gestation. Peak levels occur between 16 and 18 weeks. Maternal serum reflects that change in amniotic Alpha-Fetoprotein levels. When elevated maternal serum Alpha-Fetoprotein levels are identified, further evaluation with repeat serum Alpha-Fetoprotein levels, amniotic fluid Alpha-Fetoprotein levels, and ultrasound is warranted. Other examples of fetal body wall defects would include omphalocele and gastroschisis.
Elevated serum Alpha-Fetoprotein levels in pregnancy may also indicate multiple pregnancy, fetal distress, fetal congenital abnormalities, or intrauterine death. Low Alpha-Fetoprotein levels after correction for age of gestation, maternal weight, race, and presence of diabetes are found in mothers carrying a fetus with trisomy 21 (Down syndrome). There are other indicators of trisomy that are often performed simultaneously. See Maternal Screen Testing and Nuchal Translucency.
Alpha-Fetoprotein is also used as a tumor marker. Increased serum levels of Alpha-Fetoprotein are found in as many as 90% of patients with hepatomas. The higher the Alpha-Fetoprotein level, the greater the tumor burden. A decrease in Alpha-Fetoprotein is seen if the patient is responding to antineoplastic therapy. Alpha-Fetoprotein is not specific for hepatomas, although extremely high levels (above 500 ng/mL) are diagnostic for hepatoma. Other neoplastic conditions, such as nonseminomatous germ cell tumors and teratomas of the testes, yolk sac and germ cell tumors of the ovaries, and to a lesser extent Hodgkin disease, lymphoma, and renal cell carcinoma, are also associated with elevated Alpha-Fetoprotein levels. Testing methods for Alpha-Fetoprotein quantification include radioimmunoassay or enzyme-linked immunosorbent assay (ELISA) with a commercially available kit. Noncancerous causes of elevated Alpha-Fetoprotein levels occur in patients with cirrhosis or chronic active hepatitis.
Normal Alpha-Fetoprotein Levels
Alpha-Fetoprotein is normally found in low levels in human blood. Children normally have lower Alpha-Fetoprotein levels than adults.
Alpha-Fetoprotein levels are measured in terms of nanogram per milliliter (ng/mL) or microgram per liter (mcg/L) using SI units. No conversion is required to represent values in either the ng/mL or mcg/L. The following are the Normal Alpha-Fetoprotein Levels:
- Children younger than 1 year normally have Alpha-Fetoprotein levels less than 30 ng/mL ( 30 mcg/L).
- Adults normally have Alpha-Fetoprotein levels less than 40 ng/mL (40 mcg/L).
Causes of Alpha-Fetoprotein Fales Indications
- Fetal blood contamination, which may occur during amniocentesis, can cause increased Alpha-Fetoprotein levels.
- Multiple pregnancies can cause increased levels.
- Recent administration of Radioisotopes can affect values because results are determined by Radioimmunoassay.
Causes of High Alpha-Fetoprotein Levels in Maternal Serum
If a fetus has an open body wall defect, fetal serum Alpha-Fetoprotein leaks out into the amniotic fluid and is picked up by the maternal serum which increases Alpha-Fetoprotein in the serum. This leakage can be a result of:
- Neural Tube Defects including Anencephaly, Encephalocele, Spina Bifida, and Myelomeningocele.
- Abdominal Wall Defects including Gastroschisis and Omphalocele.
In addition to Alpha-Fetoprotein leakage. Maternal serum high levels of Alpha-Fetoprotein can be caused by:
- Multiple-fetus Pregnancy: The multiple fetuses make large quantities.
- Threatened Abortion.
- Fetal Distress or Congenital Anomalies.
- Fetal Death.
Causes of Low Alpha-Fetoprotein Levels in Maternal Serum
- Trisomy 21 (Down Syndrome).
- Fetal Wastage.
Causes of High Alpha-Fetoprotein Levels in Nonmaternal Serum
Cancers contain undifferentiated cells that may carry the surface markers of their fetal predecessors which causes high Alpha-Fetoprotein levelsin nonmaternal serum. Examples of such cancers include:
- Primary Hepatocellular Cancer (Hepatoma).
- Germ Cell or Yolk Sac cancer of the ovary.
- Embryonal Cell or Germ cell Tumor of the testes.
- Other Cancers including Stomach Cancer, Colon Cancer, Lung Cancer, Breast Cancer, or Lymphoma.
- Liver cell Necrosis including Cirrhosis and Hepatitis.