Yolk Sac Protein (AFP related)
Overview
Yolk sac protein refers to alpha-fetoprotein (AFP), a glycoprotein produced by the fetal yolk sac, fetal liver, and gastrointestinal tract that functions as a fetal serum albumin analog and fetal serum transport protein. It crosses the placenta minimally and peaks in maternal serum around 12-16 weeks gestation before declining. Elevated AFP levels in maternal serum indicate open neural tube defects, abdominal wall defects, or multiple gestations, while low levels suggest Down syndrome or other trisomies; in adults, high levels signal hepatocellular carcinoma or germ cell tumors. AFP screening is clinically essential for prenatal diagnosis of fetal anomalies and adult cancer surveillance.
Clinical Use Cases
- Second trimester maternal serum screening for neural tube defects and aneuploidy.
- Surveillance and diagnosis of hepatocellular carcinoma in cirrhosis patients.
- Monitoring nonseminomatous germ cell tumors and treatment response.
- Evaluation of acute liver injury or hepatitis.
- Prenatal diagnosis of fetal gastroschisis or omphalocele.
Specimen Types
- Maternal serum (prenatal screening).
- Adult serum or plasma.
- Amniotic fluid (for elevated MSAFP confirmation).
- Cord blood (neonatal assessment).
Measurement Methods
- Automated immunoassay analyzers (chemiluminescent, electrochemiluminescent).
- Enzyme-linked immunosorbent assay (ELISA).
- Radioimmunoassay (historical).
- Time-resolved fluor immunoassay.
Test Preparation and Influencing Factors
- No fasting required for maternal serum screening.
- Gestational age critical for interpretation (MoM multiples of median).
- Maternal obesity, smoking, or race adjust screening risk calculations.
- Recent liver injury, hepatitis, or drugs (e.g., anticonvulsants) elevate AFP.
- Multiple gestation or underestimation of gestational age falsely elevates.
- Sample hemolysis interferes with some immunoassays.
Synonyms
- Alpha-fetoprotein.
- AFP.
- Fetal albumin.
- Yolk sac tumor marker.