Alpha-Fetoprotein (AFP), α-Fetoprotein and its Significance
- This test is done on the serum of the patient.
- Serum may be stored at 2 to 8 °C for 24 hours; otherwise, freeze it.
- How to get good serum:
- Take 3 to 5 ml of blood in the disposable syringe or the vacutainer. Keep the syringe for 15 to 30 minutes at 37 °C and then centrifuge for 2 to 4 minutes to get the clear serum.
- Mother serum may be taken between 13 to 16 weeks of gestation.
- No special preparation is needed.
- AFP used as a screening test for increased risk of fetal defects like:
- Neural tube defect.
- Fetal body wall defect.
- Chromosomal abnormalities.
- This is also used as a tumor marker for various tumors, especially for liver cell carcinoma.
- AFP is the tumor marker for liver cell carcinoma and germ-cell (non-seminoma) carcinoma.
- It is ordered in the cancers of ovaries and testes.
- AFP (α-Fetoprotein) is a principal protein in the fetus, and its function in the adult is unknown.
- AFP is the most significant protein found in the second trimester of the fetus.
- This is a transport protein produced by the fetal liver with functions similar to albumin in infants and the adult body fluids.
- It is found in amniotic fluid and maternal circulation in a minimal amount due to its large molecular weight, so it cannot cross the fetoplacental circulation.
- AFP is a glycoprotein, and it is produced by the fetal liver, gastrointestinal tract, and yolk sac.
- AFP is an oncofetal protein produced by the fetal liver and yolk sac.
- Its molecular mass is 70 kD.
- It consists of a single polypeptide chain and a 4% carbohydrate.
- It is produced in large quantities in fetal life from the fetal yolk sac and liver.
- It is related both genetically and structurally to albumin.
- Gene coding for both is localized to chromosome 4q.
AFP in the fetus:
- The peak production in the fetus is by 13 weeks of gestation, and then it declines.
- The AFP may be found in the serum of the mother and amniotic fluids.
- AFP level is high in twins and multiple fetuses.
- The Modest raised level may be seen in the regenerative process of the liver.
- AFP may serve to monitor the course of liver cell carcinoma treatment.
- The AFP level is claimed to correlate with the size of the tumor in embryonal malignant teratoblastoma of testes and ovaries, so its measurement is useful for the monitoring course of the disease.
- AFP is raised in 70% of liver cancer.
- AFP role is:
- Binds and transport substances that are not water-soluble such as:
- Steroids hormones.
- Maternal AFP is lower than the expected value in Down’s syndrome.
- It is a higher level in the neural tube defect.
The normal value of AFP
|Fetal 1st-trimester peak||200 to 400|
|Adult||<8.5 (97% of the healthy population)|
|<15 (100% of the healthy population)|
|Week of gestation||ng/mL|
- Adult = <40 ng/mL
- Child < 1 year = <30 ng/mL
|Normal level||<10 ng/dL|
|Adult||<8.5 ng/dL in 97% of the healthy population|
|Adult||<15 ng/dL in 100% of the healthy population|
|Fetus first trimester||200 to 400 mg/dL|
|Cord blood||5 mg/dL|
|Amniotic fluids (13 to 16 weeks)||0.9 to 4.1 mg/dL|
|Mother serum (13 to 16 weeks)||<1.0 to 4.4 µg/dL|
- AFP greater than 1000 µg/L suggests cancer (>500 ng/mL is diagnostic of hepatoma).
Significance of AFP:
- It is a marker of Liver cell carcinoma, where greater than 500 ng/dl is found, in more than 70 to 80 % of the patient.
- It is slightly raised in Cirrhosis.
- Lower levels may be found in patients with large metastasis from Gastric or Colon tumors and patients with acute or chronic hepatitis.
- The presence and persistence of a high level above 500 ng/dl in an adult with liver disease and without any obvious GIT tumors is strongly suggestive of hepatocellular carcinoma.
- AFP may be found in the germ cell tumor.
- AFP is raised in pregnancy and neonates.
- It is raised in Choriocarcinoma and Embryonal carcinoma.
- Less commonly raised in Pancreatic carcinoma, Stomach carcinoma, Colon cancer, and lung cancer.
- AFP levels are greater in twin pregnancy.
- AFP levels are raised in the neural tube defect like spina bifida, anencephaly, myelocele, and hydrocephalus, in the mother serum. Ultrasonography can confirm this, and if the defect is serious, then pregnancy may be terminated.
- AFP level is estimated from 13 to 16 weeks of pregnancy when the fetal production of AFP is highest.
- Adult increased level indicates the hepatocellular tumor.
- Fetal increased level indicates a neural tube defect.
- Neural tube defects:
- Neural tube defects occur in the early first trimester as the spinal cord and brain develop from an embryonic structure called the neural tube.
- Failure of the neural tube to fuse leads to:
- Spina bifida is also known as meningomyelocele. There is a congenital opening in the spinal cord membrane through which the cord protrudes.
- Encephalocele is a central nervous system defect involving the brain. There is a congenital opening in the skull with protrusion of the brain tissue.
- Anencephaly when the fetus does not develop the cerebrum. There is a greatly reduced brain, particularly the cerebrum, resulting from the failure of the neural tube to close during organ formation.
- Amniotic AFP is more accurate in diagnosing neural tube defects in early gestation ( around 14 weeks) than maternal serum AFP.
- The AFP at the 8th week is very high; then there is a dip at 11 weeks and again peak at 13 weeks. Then fall in a long-linear fashion until 25 weeks.
- Before 14 weeks, AFP helps to diagnose neural tube defects. AFP is more than double the normal value.
- High-resolution ultrasound can find this defect.
- There is an increased level of AFP in the amniotic fluid.
- Normal AFP <2.5 Multiples of median (MoM).
- For the neural tube, the defect is >2.5 MoM.
- Down’s syndrome:
- It occurs in about 1 in 800 live births.
- It increases with the increasing age of the mother >35 years.
- This is a serious congenital disorder of autosomal chromosome 21 with either trisomy of the long arm or translocation or mosaic of the long arm, mostly in the q21.1 to q22.3 region.
- Clinically there is:
- Congenital heart defect.
- Muscular weakness.
- Growth and mental retardation.
- Flat facial profile with slanting eyes.
- Broad short skull.
- Broad hands with short fingers.
- An anatomical defect in the esophagus.
- Diagnosis (Tripple screening):
- AFP level is low (25% less than the normal level).
- Decreased unconjugated estriol.
- Increased chorionic gonadotropin.
- Quadruple screening when Dimeric inhibin A is added.
- Chorionic villus sampling by the 10th week.
- Karyotyping obtained from the fetal cells in amniotic fluid by the 18 to 20 weeks of gestation.
Raised AFP has been seen in:
- 100% of the metastatic liver cell carcinoma.
- Screening of liver cell carcinoma. It is raised in 80% of the cases.
- In 50% of nonseminomatous testicular cancer.
- Choriocarcinoma and embryonal carcinoma.
- Noncancerous causes are:
- Chronic active hepatitis.
- AFP levels are greater in twin pregnancy.
- In 5 to 10% of patients with cirrhosis and hepatitis.
- In alcoholic liver disease.
Decreased Maternal AFP has been seen in:
- Down’s syndrome (Trisomy 21).
- Fetal wastage.
The value of AFP to diagnose liver cancer is:
- The sensitivity of the AFP for liver cancer is 60%.
- While 40 % of liver cancer may have the normal value, so the normal level of AFP does not rule out the possibility of liver cancer.
- Elevated AFP levels may be seen in other conditions as well.
- Although the FDA does not recommend AFP for cancer screening, it is used to detect and monitor liver cell carcinoma and other tumors like testes and ovaries, retroperitoneum, and mediastinum.
AFP as a tumor marker for:
- Liver cell carcinoma.
- Testicular germ cell tumors.
- Gastric carcinoma.
- Pancreatic carcinoma.
- Other possible causes of raised AFP:
- Nonseminomatous germ cell tumor.
- Teratomas of the testes.
- Yolk sac tumors.
- Germ cell tumors of the ovaries.
- Sometimes may be seen in:
- Hodgkin’s lymphoma.
- Renal cell carcinoma.