Alpha-Fetoprotein (AFP) is a protein detected in the blood, and measuring its levels serves as a common diagnostic tool in various medical contexts. It is typically measured through a simple blood test. Understanding these levels, whether high or low, provides insights into different physiological and pathological states.
What Alpha-Fetoprotein Is
Alpha-Fetoprotein is a glycoprotein primarily synthesized by the fetal liver and yolk sac during prenatal development. This protein plays several roles in fetal life, including maintaining plasma oncotic pressure and transporting various substances like fatty acids to developing cells. It is considered the fetal equivalent of serum albumin, a major protein in adult blood.
AFP levels are naturally elevated in fetuses and newborns, reflecting its active production during growth and development. Following birth, AFP production rapidly declines, and its levels become very low in healthy children and adults, ranging from 0 to 40 ng/mL. While its specific function in adult humans is not fully understood, trace amounts may persist.
AFP Levels in Pregnancy Screening
Maternal serum AFP (MSAFP) testing is a routine component of prenatal screening, often included in comprehensive assessments like the triple or quad screen, performed between 15 and 20 weeks of pregnancy. This screening assesses the risk of certain birth defects and genetic conditions in the developing fetus. Fetal AFP passes through the placenta into the mother’s bloodstream for measurement.
Elevated AFP levels in maternal blood can suggest an increased risk of neural tube defects, such as spina bifida (incomplete spinal column closure) or anencephaly (incomplete brain and skull development). Openings in the fetal body allow more AFP to leak into the amniotic fluid and then into the mother’s circulation. Conversely, lower than expected AFP levels may indicate an increased risk for chromosomal abnormalities, including Down syndrome (Trisomy 21) or Edwards syndrome (Trisomy 18), where there is an extra copy of a chromosome.
AFP screening is not a definitive diagnostic test; rather, it identifies pregnancies that may require further evaluation. If AFP levels are abnormal, additional tests like ultrasound or amniocentesis may be recommended to confirm or rule out a suspected condition. Several factors can influence MSAFP levels, including gestational age (the most significant determinant), multiple pregnancies, maternal weight, and race.
AFP Levels as a Tumor Marker
Beyond pregnancy screening, AFP serves as a biomarker for certain cancers in adults. It is primarily associated with hepatocellular carcinoma (HCC), a common liver cancer, and specific germ cell tumors like testicular and ovarian cancers. Elevated AFP levels assist in the initial diagnosis of these cancers.
AFP levels also help monitor cancer treatment effectiveness and detect potential recurrence. A decrease in AFP levels following therapy indicates a positive response, while a rise suggests disease progression or return. For instance, the AFP-L3 subtype is useful in diagnosing HCC, with levels above 10% of total AFP indicating a higher risk.
Elevated AFP levels are not exclusive to cancer. Other non-cancerous liver conditions, such as cirrhosis (scarring) or chronic hepatitis, can also cause elevated AFP levels due to liver cell regeneration or inflammation. Therefore, AFP is not used as a standalone diagnostic test for cancer. Instead, it is interpreted with other clinical findings, imaging studies (like ultrasound or CT scans), and biopsies for a comprehensive assessment.