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Alpha-fetoprotein

Alpha-fetoprotein (AFP, α-fetoprotein; also sometimes called alpha-1-fetoprotein, alpha-fetoglobulin, or alpha fetal protein) is a protein found in the blood that is produced by a fetus’s liver and also is produced by some cancer cells, making it a helpful marker used to detect and distinguish some types of cancer.

Background

Alpha-fetoprotein (AFP) is a globulin protein secreted by the yolk sac, gastrointestinal tract and liver cells in the developing fetus, in adulthood it is secreted in the liver. Therefore, the highest levels of AFP are found in the blood during pregnancy and in those individuals with liver dysfunction or cancer. Although AFP is not a specific blood marker for malignancies, its measurement may be used to monitor the effectiveness of surgical and chemotherapeutic management of hepatomas (liver tumors) and germ cell tumors (teratomas and yolk sac tumors).

How the test is performed

A blood sample is needed. Most of the time, blood is typically drawn from a vein located on the inside of the elbow or the back of the hand. Then the specimen is analyzed in the laboratory typically using immunoassay. Normal adult levels are usually achieved by the age of 8 to 12 months. Approximately 97 to 98% of the healthy population have AFP levels less than 8.5 ng/mL.

In pregnancy, measurement of AFP is usually performed from week 16 to 20 to help identify fetal abnormalities and is usually strongly recommended for women with a family history of birth defects, pregnant women older than 35 years, those who have diabetes mellitus or have been taking any medicined that could be toxic to the fetus. In other cases, AFP detection helps to diagnose and monitor some malignancies including liver cancer and germ cell tumors (AFP tumor marker test).

AFP in pregnancy

The test is performed when a fetal abnormality and especially a defect of the neural tube in the fetus is suspected according to the results of ultrasonography examination. Between the 16 and 18th weeks of gestation, AFP level is measured in a maternal serum triple or quadruple screening test. The amount of AFP in the amniotic fluid may also be measured if the amniotic fluid is collected during amniocentesis.

During pregnancy AFP values are the following:

  • 12 weeks gestation <42 µg/mL;
  • 14 weeks gestation <35 µg/mL;
  • 16 weeks gestation <29 µg/mL;
  • 18 weeks gestation <20 µg/mL;
  • 20 weeks gestation <18 µg/mL;
  • 22 weeks gestation <14 µg/mL;
  • 30 weeks gestation <3 µg/mL;
  • 35 weeks gestation <2 µg/mL;
  • 40 weeks gestation <1 µg/mL;

The level of alpha-fetoprotein in a pregnant woman may be increased in the case of:

  • anencephaly (absence of a major portion of the brain, skull, and scalp that occurs during embryonic development);
  • cystic fibrosis (a genetic disorder);
  • duodenal atresia (congenital absence or complete closure of a portion of the lumen of the duodenum);
  • esophageal atresia (congenital absence or complete closure of a portion of the lumen of the esophagus);
  • obstruction of the ureter with hydronephrosis;
  • fetal death;
  • meningomyelocele (herniation of the spinal cord and its meninges through a defect in the vertebral arch);
  • multiple pregnancy;
  • nephrosis (congenital);
  • neural tube defects;
  • spina bifida (defect of the vertebral arch);
  • omphalocele (a birth defect of the abdominal wall);
  • Turner syndrome (a condition in which a female is partly or completely missing an X chromosome).

Lower maternal serum AFP concentrations may indicate disorders in the unborn child including trisomy of chromosome 18 (Edwards syndrome) or 21 (Down syndrome).

It should be noted that in most cases abnormal AFP levels may be a sign of wrong due date calculation rather than any abnormality in an unborn child – as AFP levels change through pregnancy a bit higher or lower AFP may suggest that a baby is younger or older than suggested by due date calculations.

AFP in cancer

AFP test is required to diagnose liver malignancies (hepatocellular carcinoma, primary hepatoma) as well as monitor treatment response in hepatoma and germ cell neoplasms and respectively assess prognosis. However, AFP is a non-specific tumor marker and can be increased in lymphomas and renal cell carcinomas as well. In high-risk patients, AFP values between 100 and 350 ng/mL suggest hepatocellular carcinoma.

AFP in non-cancer conditions

AFP measurement is also done to follow up with patients with chronic hepatitis (alcoholic, viral) or liver cirrhosis. AFP levels also appear to increase in individuals with ataxia-telangiectasia syndrome (neurodegenerative, autosomal recessive disease) and hereditary tyrosinemia (metabolic disorder).

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