[Clinical significance of hepatitis B surface antigen quantification in chronic hepatitis B]

Since the discovery of HBsAg in the early 1960s, presence of HBsAg in serum has only served to diagnose hepatitis B. Recent development in the quantitative measurement of serum HBsAg has enabled us to improve our understanding on the management of chronic hepatitis B. The surface antigen (sAg) level is at its highest in immune tolerance phase and decreases to the lowest level in immune control/inactive phase when HBeAg is cleared from the serum. Combination of serum sAg titer less than 1,000 IU/mL and serum HBV DNA less than 2,000 IU/mL can identify true inactive carrier from e antigen (eAg) negative hepatitis with diagnostic accuracy of 95%. During the natural course of chronic hepatitis B, changes or absolute level of sAg less than certain level can predict spontaneous sero-clearance of HBsAg. Although the decline of sAg is very slow in interferon (IFN)/pegylated interferon (PEG-IFN) or oral nucleos(-t)ide treated patients, interferon based therapy results in a greater decrease of sAg level and sAg loss. Lack of any decline in sAg titer during PEG-IFN therapy could identify the group of patients who do not response to IFN/PEG-IFN therapy. With the aid of serum HBV DNA, quantitative measurement of serum HBsAg level can be used to optimize the management of chronic hepatitis B in our daily practice.