Alpha-fetoprotein
From Wikipedia, the free encyclopedia
| ICD-10 | R77.2, Z36.1 |
|---|---|
| ICD-9 | V28.1 |
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alpha-fetoprotein
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| Identifiers | |
| Symbol(s) | AFP HPAFP |
| Entrez | 174 |
| OMIM | 104150 |
| RefSeq | NM_001134 |
| UniProt | P02771 |
| Other data | |
| Locus | Chr. 4 q11-q13 |
Alpha-fetoprotein (AFP) is a protein that is normally only produced in the fetus during its development. It is a normally produced by the liver and yolk sac of the fetus. AFP levels decrease soon after birth and probably has no function in normal adults. It binds the hormone estradiol to keep it from affecting the fetal brain. Its measurement during pregnancy has been useful to detect certain abnormalities - specifically, if high levels of AFP are found in amniotic fluid, it can indicate a developmental defect in the baby. In some patients who are not pregnant a tumor can produce AFP, thus it can be used as a tumor marker.
[edit] Structure and levels
AFP is a glycoprotein of 590 amino acids and a carbohydrate moiety that is normally produced by the fetal yolk sac, the fetal gastrointestinal tract, and eventually by the fetal liver. Highest fetal serum levels of AFP are reached at the end of the first trimester and then fall. As AFP is excreted into the amniotic sac through fetal urination, amniotic fluid levels tend to mirror fetal serum levels. In contrast, maternal levels are much lower but continue to rise until about week 32. Levels are much higher in amniotic fluid.
[edit] AFP screening
Maternal serum AFP tests need to be interpreted according to the gestational age, as levels rise until about 32 weeks gestation. Typically, such measurements are done in the middle of the second trimester (14-16 weeks). Elevated levels are seen in multiple gestation as well as in a number of fetal abnormalities, such as neural tube defects including spina bifida, anencephaly, and abdominal wall defects. Other possibilities are errors in the date of the gestation or fetal demise. In contrast, low levels of maternal serum AFP are associated with Down syndrome and trisomy 18. Diabetic patients also have lower levels. Patients with abnormal levels need to undergo detailed obstetric ultrasonography. The information is then used to decide whether to proceed with amniocentesis.
Typically AFP measurements are done as part of the triple test, a screening program in pregnant women which also looks at hCG and estriol levels. Genetic counseling is usually offered when the AFP test result is screen positive.
AFP testing for neural tube defects has been increasingly replaced by ultrasound screening during pregnancy.
[edit] Tumor marker
Like all tumor markers, the detection of AFP by itself is not diagnostic of anything, although if it is detected it is certainly advisable to rule out the diseases could cause levels to rise. The primary reason tumor markers are used are to measure the success of a treatment (e.g. chemotherapy), if levels of AFP are going down, it is an indication that a disease is improving. New research exhibits that an isoform of AFP which binds Lens culinaris agglutinin (AFP-L3) can be particularly useful in early identification of aggressive tumors associated with hepatocellular carcinoma (HCC).
AFP is the main tumor marker (along with HCG) to diagnose testicular cancer and its values over time can have significant effect on the treatment plan.
