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.