Alpha-Fetoprotein

Alpha fetoprotein (AFP) is a 70 kDa glycoprotein made by the foetal yolk sac, liver and gut. It is the main foetal serum protein and falls to under 10 microg/L within the first year of life.

In adults it is re-expressed by hepatocellular carcinoma (HCC), germ cell tumours, regenerating liver and some other tumours. In pregnancy it appears in maternal serum and amniotic fluid and screens for foetal neural tube and abdominal wall defects.

It is measured by immunoassay in serum and reported in microg/L (ng/mL) or kU/L (1 microg/L is about 0.83 kU/L).

The test is a tumour marker for HCC and non-seminomatous germ cell tumours, and a screening marker in pregnancy. It supports diagnosis, staging, prognosis and monitoring of treatment.

Hepatocellular carcinoma is the sixth commonest cancer and the third leading cause of cancer death worldwide, with over 800 000 deaths a year. Most arise in cirrhosis, from hepatitis B and C, alcohol and metabolic liver disease, and surveillance is offered to cirrhotic patients every 6 months. Neural tube defects affect 1 in 1000 pregnancies. Testicular germ cell tumours affect 4 to 10 per 100 000 men.

Indications.

• Surveillance of patients with cirrhosis or chronic hepatitis B (with ultrasound).
• Liver mass of unknown cause.
• Testicular or extragonadal midline mass in a young adult.
• Monitoring after treatment of HCC or germ cell tumour.
• Second trimester maternal serum screening.
• Hepatoblastoma in children.

Specimen. Serum. No fasting is needed. Stable for days at 2 to 8 °C. In pregnancy, record gestational age, maternal weight, number of foetuses, diabetes and ancestry for risk calculation.

Reference range. Non-pregnant adults: under 10 microg/L (under 8 kU/L). Neonates have high values (up to 100 000 microg/L at birth in term infants), falling to adult range by 6 to 12 months.

Hepatocellular carcinoma.

• Surveillance: a level over 20 microg/L raises concern, and the sensitivity of AFP alone at this cut off is only 40 to 65%, with specificity of 80 to 90%. Combined with ultrasound, detection of early stage tumours improves.
• Diagnostic: a level over 400 microg/L in a patient with a liver mass and cirrhosis is highly suggestive. Values over 1000 microg/L are nearly diagnostic.
• Prognostic: levels over 400 microg/L predict a poorer outcome and microvascular invasion. A rapid rise and AFP-L3 over 10 to 15% (the lens culinaris reactive fraction) and des-gamma-carboxy prothrombin (DCP or PIVKA-II) also predict tumour behaviour.
• Monitoring: AFP should fall within 4 to 8 weeks of effective resection or ablation (half life about 5 days), and a rise after treatment signals recurrence.

Non-seminomatous germ cell tumours. Raised in yolk sac tumour (very high), embryonal carcinoma and mixed tumours. Pure seminoma does not raise AFP, so a raised AFP in a testicular mass labelled seminoma indicates a non-seminomatous component. Together with hCG and LDH, it gives the S category: S1 (AFP under 1000 microg/L), S2 (1000 to 10 000 microg/L), S3 (over 10 000 microg/L). Half life after orchidectomy is 5 to 7 days, and a slow decline suggests residual disease. Marker negative disease is monitored by imaging.

Other tumours. Hepatoblastoma (levels very high in infancy), gastric, pancreatic and biliary cancers, lung and colorectal cancers with liver metastases (usually modest rises), and ovarian germ cell tumours (yolk sac, Sertoli-Leydig).

Benign causes of raised AFP. Acute and chronic hepatitis (typically under 100 microg/L, and up to 500 in active flares), cirrhosis, liver regeneration after injury, pregnancy, ataxia telangiectasia (persistent), hereditary tyrosinaemia type 1, and hereditary persistence of AFP.

Maternal serum screening.

• Unexplained high AFP (over 2.0 to 2.5 multiples of the median) at 15 to 20 weeks suggests an open neural tube defect, abdominal wall defect, multiple pregnancy, wrong dates, placental abnormality, foetal death or maternal causes. Ultrasound confirms.
• Low AFP (under 0.5 multiples of the median) is associated with Down syndrome, trisomy 18, wrong dates, molar pregnancy, and maternal obesity or diabetes (adjusted for weight).
• AFP is combined with hCG, unconjugated oestriol and inhibin A in the quadruple test.

Amniotic fluid. AFP and acetylcholinesterase confirm an open neural tube defect.

Critical values and findings needing immediate action.

• AFP over 400 microg/L with a liver mass: urgent multiphase imaging and specialist referral.
• Raised AFP in a young man with a testicular mass or midline tumour: urgent oncology referral, since cure rates are high with prompt treatment.
• Rising AFP during surveillance of cirrhosis, even if ultrasound is negative.
• Persistent or rising AFP after treatment of germ cell tumour or HCC.
• High maternal serum AFP with ultrasound showing a neural tube or abdominal wall defect.

Interferences and pitfalls.

• AFP is normal in 30 to 40% of HCC at diagnosis, especially small tumours, so a normal value does not exclude cancer.
• Rises occur with hepatitis flares, and in chronic hepatitis C they fall with antiviral cure, so a modest rise should be interpreted with the clinical state.
• Heterophile antibodies and the hook effect (very high values giving falsely low readings) can occur, and dilution corrects the latter.
• Biotin interferes with some immunoassays.
• Different assays give different results, and trend uses the same method.
• AFP-L3 and DCP add to AFP but are not available everywhere.
• Maternal screening needs accurate gestational age, and interpretation uses multiples of the median adjusted for weight, ancestry and diabetes.

Special groups.

• Neonates and infants: values are high in the first months, and interpretation requires age specific ranges.
• Children: hepatoblastoma, yolk sac tumours and sacrococcygeal teratoma raise AFP, and the half life in infancy is longer.
• Pregnancy: levels rise through the second trimester, and a high value is not diagnostic.
• Older adults with cirrhosis: surveillance continues while treatment is possible.
• Regions: high rates of hepatitis B and aflatoxin exposure increase HCC in sub-Saharan Africa and East Asia, and AFP may be the only available marker.

Follow up tests. Liver ultrasound, multiphase CT or MRI, hCG and LDH, testicular ultrasound, AFP-L3 and DCP, hepatitis B and C serology and viral load, liver function tests, INR and platelets, fibrosis assessment, tissue biopsy, maternal ultrasound and amniocentesis (AFP and acetylcholinesterase), and chest and abdominal CT for staging.