AFP Blood Test: Alpha-Fetoprotein Levels Explained

Understanding Alpha-Fetoprotein (AFP)

When a patient presents with unexplained weight loss or during a routine prenatal visit, the Alpha-Fetoprotein (AFP) test frequently emerges as a critical diagnostic tool. This protein, essential for fetal development, can reveal significant insights into both maternal and adult health when measured in the blood.

What is Alpha-Fetoprotein?

Alpha-fetoprotein is a glycoprotein primarily produced by the fetal liver and yolk sac during pregnancy. After birth, levels plummet and remain very low in healthy adults. The internationally recognized LOINC standard name for this measurement is "Alpha-fetoprotein (AFP) [Mass/volume] in Serum or Plasma".

Why is the AFP Test Ordered?

In my clinical practice, I often request an AFP test for two principal scenarios: evaluating fetal well-being during pregnancy and screening for or monitoring specific cancers, particularly hepatocellular carcinoma and germ cell tumours.

AFP in Prenatal Screening

Between 15 and 20 weeks of pregnancy, AFP is measured as part of a multiple-marker serum screen. This helps assess the risk of neural tube defects, such as spina bifida, and chromosomal abnormalities like Down syndrome. It's a screening test, not a definitive diagnosis.

AFP in Oncology

For adults, a rising AFP can be a red flag for primary liver cancer or germ cell tumours of the ovaries or testes. We also use serial AFP measurements to monitor treatment response and check for cancer recurrence.

Normal AFP Reference Ranges

AFP levels vary significantly with age, pregnancy, and health status. The table below provides general guidance, but always consult your lab's specific reference intervals.

*MoM: Multiples of the Median, a standardised way to report prenatal screening results.

Interpreting AFP Test Results

What Does a High AFP Level Mean?

An elevated AFP in non-pregnant adults necessitates further investigation. While it raises suspicion for cancers like hepatocellular carcinoma, benign conditions such as chronic viral hepatitis, cirrhosis, or even liver regeneration can cause increases. My patients with active hepatitis often show transient, mild AFP elevations.

Is Low AFP Dangerous?

Low AFP is typically normal in adults. During pregnancy, a low AFP level may be associated with a higher risk of Down syndrome, but it is always interpreted in conjunction with other markers like hCG and estriol.

Can You Have Liver Cancer with a Normal AFP?

Yes. Approximately 20-30% of hepatocellular carcinomas do not produce significant amounts of AFP. Therefore, a normal AFP result does not completely rule out liver cancer, especially in high-risk individuals with cirrhosis.

How the Test is Performed

The AFP test involves a simple venipuncture to draw a blood sample. Fasting is not usually required, but your doctor may provide specific instructions based on the clinical context.

Next Steps After an Abnormal Result

An out-of-range AFP result should be discussed with your healthcare provider. Follow-up may include imaging studies like an abdominal ultrasound or fetal anatomy scan, more specific blood tests (e.g., Lens culinaris agglutinin-reactive AFP fraction for liver cancer), or invasive procedures like amniocentesis in pregnancy.

Frequently Asked Questions

How often is AFP checked for cancer surveillance?

For patients at high risk of hepatocellular carcinoma, such as those with cirrhosis, we typically recommend AFP testing and ultrasound every six months.

Are there false positives with the AFP test?

Yes. Factors like incorrect pregnancy dating, multiple gestation, or maternal weight can affect prenatal AFP results. In adults, liver inflammation can temporarily elevate AFP without indicating cancer.

What other tests accompany AFP in prenatal screening?

AFP is commonly part of the "quad screen," which also measures human chorionic gonadotropin (hCG), unconjugated estriol (uE3), and inhibin A to improve detection accuracy for certain conditions.