HBV DNA is the deoxyribonucleic acid (i.e., hepatitis B virus gene) of the hepatitis B virus. HBV DNA is the most direct, specific and sensitive indicator of HBV infection, and higher HBV DNA quantification indicates more viral replication and greater infectivity. The serum test for hepatitis B surface antigen (HBsAg) is the traditional marker for determining hepatitis B virus infection. With the improvement of testing technology, the quantitative serum HBsAg test and its clinical significance have become one of the hot spots in the field of hepatitis B research. It is now clear that serum HBsAg levels can indirectly respond to the amount and transcriptional activity of the template for hepatitis B virus replication (cccDNA, which is the main reason why hepatitis B virus is difficult to be completely cleared) in hepatocytes. serum HBsAg levels are closely related to the natural history of chronic hepatitis B virus infection, and the clinical significance of its quantitative testing is: 1) to help distinguish hepatitis B e antigen (HBeAg)-negative chronic hepatitis B versus inactive HBsAg carriers; 2) help predict durable virologic response to antiviral agents, especially interferon therapy – serum HBsAg tends to decline rapidly in patients who are complete responders to antiviral therapy, while HBsAg declines relatively slowly in patients who are incomplete responders.