Alpha-fetoprotein (AFP) is a protein naturally produced by the body, serving as a significant marker in medical diagnostics. Its levels can be measured in the blood to provide insights into various physiological states. While AFP is prominently present during fetal development, its presence in adults, even at very low levels, makes it a valuable tool for screening and monitoring certain conditions. The AFP serum test helps healthcare professionals assess potential risks and guide further investigation.
What Alpha-Fetoprotein Is
Alpha-fetoprotein is a glycoprotein primarily synthesized by the fetal liver and yolk sac during prenatal development. This protein is the most abundant plasma protein in the human fetus, playing a role similar to albumin by helping transport substances like fatty acids and hormones within the fetal bloodstream. After birth, AFP levels typically decrease significantly, reaching very low concentrations in healthy children and non-pregnant adults.
The AFP gene, located on chromosome 4q25, encodes this 70-kDa single polypeptide chain, which contains a small percentage of carbohydrates. While its exact function in adults remains unclear, its re-expression in certain conditions offers diagnostic utility. The protein exhibits a triplicate domain structure, with its half-life in the bloodstream being approximately four to five days.
Key Reasons for AFP Testing
An AFP serum test is primarily utilized in two distinct clinical contexts: prenatal screening and as a tumor marker. In pregnancy, the test is a component of prenatal screening, often included in a “quad screen” or similar multiple marker tests. This screening aims to identify potential risks for neural tube defects, such as spina bifida and anencephaly, where elevated AFP levels may occur. The test also screens for chromosomal abnormalities like Down syndrome and Edwards syndrome, which are typically associated with lower AFP levels.
For non-pregnant individuals, the AFP test functions as a tumor marker. It is particularly useful in the diagnosis and monitoring of certain cancers, most notably hepatocellular carcinoma, a type of liver cancer. Elevated AFP levels can also indicate the presence of germ cell tumors, which originate from reproductive cells, such as those found in the testicles or ovaries.
Interpreting AFP Test Results
Interpreting AFP test results involves comparing the measured level to established normal ranges, which vary depending on the individual’s condition. For non-pregnant adults, normal AFP levels are typically very low, generally ranging from 0 to 10 ng/mL, though some laboratories define the upper limit of normal as 40 ng/mL. During pregnancy, AFP levels naturally increase as the fetus develops, with typical ranges between 10 and 150 ng/mL during weeks 15 to 20 of gestation. These levels are often reported as “multiples of the median” (MoM) to account for gestational age.
Elevated AFP levels in pregnant individuals can suggest several possibilities. Higher than expected levels may indicate an increased risk of neural tube defects like spina bifida or anencephaly, or abdominal wall defects where fetal organs are exposed. Elevated levels can also occur in multiple pregnancies, such as twins, or if the estimated due date is inaccurate, meaning the pregnancy is further along than initially thought. In non-pregnant individuals, high AFP levels are associated with hepatocellular carcinoma, germ cell tumors, and certain other cancers like those of the stomach or lung. Non-cancerous conditions, including cirrhosis, hepatitis, and other liver injuries, can also lead to elevated AFP.
Conversely, lower than expected AFP levels during pregnancy can also be a cause for further investigation. Abnormally low maternal serum AFP is associated with an increased risk of chromosomal abnormalities, particularly Down syndrome and Edwards syndrome. Similar to elevated levels, an incorrect estimation of gestational age can also result in seemingly low AFP readings if the pregnancy is less advanced than calculated. It is crucial to understand that an abnormal AFP result, whether high or low, does not confirm a diagnosis but rather indicates a need for additional, more specific tests.
Beyond the Initial Result
Several non-disease related factors can influence AFP levels, leading to results outside the typical range. An incorrect calculation of gestational age is a common reason for seemingly abnormal AFP levels in pregnancy, as AFP concentrations change significantly throughout fetal development. Carrying multiple fetuses, maternal weight, race, and the presence of conditions like diabetes can affect AFP concentrations. Certain benign liver conditions, such as acute or chronic hepatitis and cirrhosis, can also cause elevated AFP levels in non-pregnant individuals.
When an AFP test yields an abnormal result, healthcare providers will typically recommend follow-up testing to clarify the findings. For pregnant individuals, this may include a detailed ultrasound to confirm gestational age and examine fetal anatomy for structural abnormalities. An amniocentesis, which analyzes amniotic fluid, might also be suggested for a more accurate diagnosis of chromosomal conditions or neural tube defects. For non-pregnant individuals, further investigations could involve imaging studies such as CT scans or MRIs, and sometimes a liver biopsy, to determine the underlying cause of elevated AFP. It is important to discuss all test results with a healthcare provider, who can offer a comprehensive interpretation based on an individual’s complete medical history and guide the appropriate next steps.