Alpha-fetoprotein
Alpha-fetoprotein (AFP) is a protein encoded by the AFP gene.
The AFP gene is located on the q arm of chromosome 4 (4q25).
Maternal AFP serum level is used to screen for Down syndrome, neural tube defects, and other chromosomal abnormalities.
AFP is a major plasma protein produced by the yolk sac and the fetal liver during fetal development.
It is thought to be the fetal analog of serum albumin.
It is a glycoprotein of 591 amino acids
and a carbohydrate moiety.
Its function in adults is unknown.
AFP is the most abundant plasma protein found in the human fetus.
Maternal plasma levels peak near the end of the first trimester.
AFP plasma levels begin decreasing prenatally at that time, then decrease rapidly after birth.
Normal adult levels in the newborn are usually attained by the age of 8 to 12 months.
AFP may protect the fetus from maternal estradiol that would otherwise have a masculinizing effect on the fetus, but its exact role is still controversial.
In pregnant women, fetal AFP levels can be monitored in the urine of the pregnant woman.
AFP is quickly cleared from the mother’s serum via her kidneys.
Maternal urine AFP correlates with fetal serum levels, although the maternal urine level is much lower than the fetal serum level.
AFP levels rise until about week 32.
Maternal serum alpha-fetoprotein screening is performed at 16 to 18 weeks of gestation.
The normal range of AFP for adults and children is reported as under 50, under 10, or under 5 ng/mL.
At birth, AFP levels are four or more orders of magnitude above the normal range, that decreases to a normal range over the first year of life.
AFP is measured in pregnant women through the analysis of maternal blood or amniotic fluid.
AFP levels are used as a screening test for certain developmental abnormalities, such as aneuploidy.
Low levels associated with Down syndrome.
The serum AFP level is elevated in certain tumors, and is used as a biomarker to follow these diseases.
Developmental birth defects associated with elevated AFP:
Omphalocele
Gastroschisis
Neural tube defects: ↑ α-fetoprotein in amniotic fluid and maternal serum.
Tumors associated with elevated AFP:
Hepatocellular carcinoma
Metastatic disease affecting the liver
Nonseminomatous germ cell tumors
Yolk sac tumor
In Ataxia telangiectasia an elevated AFP is used as one factor in diagnosis.
AFP helps to differentiate seminomatous and nonseminomatous tumors: if the pathology is pure seminoma, and the AFP is elevated, the tumor is treated as a nonseminomatous tumor because it nonseminomatous components.