**2.1 Alpha-fetoprotein (AFP)**

Alpha-fetoprotein (AFP) is a large serum glycoprotein that is synthesized in the liver that occurs during fetal life is repressed during adulthood [5]. Therefore, AFP levels often diminish rapidly after birth and remain low throughout adulthood. Since AFP was discovered in the serum of HCC patients in 1964 [6], it has been regarded as the most useful serum protein for patients at risk for HCC [7–9]. However, the sensitivity and specificity of using AFP for early HCC detection are widely variable as elevated AFP levels are also observed in many other cancers [10]. In addition, AFP levels are below the detection limit in small liver tumors, while it can be above the detection limit when the tumor is large, producing an AFP-negative HCC. AFP is considered to have a screening role in HCC but its role is limited since it does not allow to distinguish between cancerous lesions and some other benign liver damage pathologies, hence causing a high proportion of false positives and false negatives. Patients with hepatitis still have high AFP level even without liver tumors. The positive predictive value of AFP for detecting HCC is 70% for people with hepatitis viruses and 94% for those without. Therefore, AFP is more effective in detecting HCC in cases without hepatitis viruses.

According to the 2010 recommendations of the American Association for the Study of Liver Diseases (AASLD) for the diagnosis and treatment of HCC, the effectiveness of AFP as a test to diagnose HCC was lower than expected. AFP is also increased in biliary carcinoma in the liver or metastases from colon cancer. Biliary cancer in the liver is also quite common in cirrhotic patients, although the incidence of this disease is lower than that of HCC. The fact that these two liver cancers are common in cirrhosis makes it necessary to identify accurately the disease. Because AFP may increase in many cases other than HCC, it is no longer recommended to be used in Europe and the Americas for its diagnosis. The current diagnosis of HCC is based on imaging and histopathology [11]. The Asia Pacific Association for the Study of the Liver (APASL) also stated that AFP alone is not recommended to diagnose HCC. When combined with other methods, the diagnosis threshold of AFP was 200 ng/ml (**Table 1**) [12].

#### **2.2 AFP heterogeneity**

AFP exists as three glycoforms, each of them having a different binding capability to lectin *Lens culinaris* agglutinin (LCA): AFP-L1 (non-binding fraction), AFP-L2 (weak binding fraction), and AFP-L3 (binding fraction). AFP-L1 is increased in early stages of liver disease progression, AFP-L2 has an intermediate affinity for lectin and is a major component during pregnancy because it is derived from the yolk sacs. AFP-L3 is only elevated in patients with HCC because it is solely produced by cancer cells, making it a specific biomarker for HCC [13, 14]. However, the drawback of AFP-L3 is that it can only be detected if AFP levels are >20 ng/ml.

AFP-L3 immunoassay sensitivity has been further improved by higher sensitivity analytical methods and advanced microfluidics-based separation science [15]. "Highly sensitive AFP-L3" (hs-AFP-L3) obtained significantly better results than conventional AFP-L3, even when patients had a single and/or small HCC tumor. The sensitivity and specificity of hs-AFP-L3 were 57% and 63.5%, and 40.4% and 81.1% for conventional AFP-L3 [16]. These results make hs-AFP-L3 a valuable biomarker for detecting early-stage HCC (**Table 1**).


*Circulating Biomarkers for Early Diagnosis of Hepatocellular Carcinoma DOI: http://dx.doi.org/10.5772/intechopen.98483*

**Abbreviation:** *n.a.: not applicable; AFP, alpha-fetoprotein; GP73, Golgi protein 73; GPC3, Glypican-3; OPN, Osteopontin; SCCA, squamous cell carcinoma antigen; DCP, Des-*γ*-carboxyprothrombin; GGT, Gammaglutamyltransferase; AFU, Alpha L fucosidase; TGF-*β*1, Transforming growth factor-*β*; IGF-II, insulin-like growth factor-II; HGF, Hepatocyte growth factor.*

#### **Table 1.**

*Diagnostic performance of biomarkers for HCC.*
