Alpha-fetoprotein

From Wikipedia, the free encyclopedia

**Elevated alphafetoprotein**
*Classification & external resources*
ICD-10	R77.2, Z36.1
ICD-9	V28.1

alpha-fetoprotein
Identifiers
Symbol	AFP HPAFP
HUGO	317
Entrez	174
OMIM	104150
RefSeq	NM_001134
UniProt	P02771
Other data
Locus	Chr. 4 q11-q13

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Alphafetoprotein (AFP) is a protein produced in the developing embryo and fetus. In humans, AFP levels decrease gradually after birth, reaching adult levels by 8 to 12 months. Normal adult AFP levels are low, but detectable; however, AFP has no known function in normal adults. In normal fetuses, AFP binds the hormone estradiol. AFP is measured in pregnant women, using maternal blood or amniotic fluid, as a screening test for a subset developmental abnormalities, principally open neural tube defects. It is also measured in pregnant women, other adults, and children, to detect a subset tumors, principally endodermal sinus tumors.

1 Structure and levels
2 History
3 AFP tests
- 3.1 Tests performed on serum
  - 3.1.1 Maternal serum
  - 3.1.2 Tumor marker
- 3.2 Tests performed on amniotic fluid
4 Serial monitoring
5 References
6 External links

[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. Levels of AFP in fetal serum rise until the end of the first trimester of gestation and then fall. Because the fetus excretes AFP into its urine, amniotic fluid levels of AFP tend to mirror fetal serum levels. In contrast, maternal serum levels of fetal AFP are much lower but continue to rise until about week 32.

[edit] History

LabCorp, a large US clinical laboratory testing company, began offering AFP screening tests in the early 1980's.^[1]

[edit] AFP tests

There are two categories of AFP tests: tests performed on serum (blood plasma), and tests perfomed on amniotic fluid. Tests performed on serum are further categorized by the reason for performing the test: maternal serum, adult tumor marker, and pediatric tumor marker.

[edit] Tests performed on serum

For these tests, the patient visits a phlebotomy lab to have a blood sample drawn. Usually, this requires that the patient (or guardian) first obtain a written order from the patient's physician. In the US, an alternative is to use MyMedLab.

The standard is a quantitative test, reporting a measured concentration of AFP in the sample, but there is also a less expensive qualitative test, reporting only that the concentration is normal or high. The qualitative test is appropriate only in some circumstances.

The resulting test report should specify the assay method and equipment used, and the report of a quantitative test should also provide a reference range for the test result. Many laboratories report reference ranges that are based on all other samples tested in that laboratory, necessarily including samples with abnormal AFP concentrations due to disease. Superior reference ranges are produced by research on healthy subjects.

[edit] Maternal serum

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 a screening program in pregnant women which also looks at hCG and estriol levels (the triple test), or hCG, estriol, and Inhibin (the quad test). Genetic counseling is usually offered when the test result is screen positive.

[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.

[edit] Tests performed on amniotic fluid

[edit] Serial monitoring

If repeated measurements of AFP are needed, some clinical testing laboratories provide a special reporting mechanism, serial monitor that links data pertaining to the person being tested. This requires a unique identifier for the person. In the United States commonly a Social Security number is used for this. One important function of this mechanism is to ensure that each sample is tested using the same assay. For AFP, several different assays, based on different technologies, are available. AFP measurements obtained using different assays are not comparable unless special calculations are performed.