- Hepatitis B surface antigen (HBsAg) is a distinctive serological marker of acute or chronic hepatitis B infection.

- HBsAg is the first antigen to appear following infection with HBV and is generally detected 1-10 weeks after the onset of clinical symptoms.

- HBsAg assays are routinely used to diagnose suspected HBV infection and monitor the status of infected individuals to determine whether the infection has resolved or the patient has become a chronic carrier of the virus.

- In patients that recover from HBV infection, HBsAg is undetectable 3-5 months after the onset of infection.

- In patients with chronic HBV infection, HBsAg remains detectable for life.

- Prenatal HBsAg screening has been recommended so that newborns from HBV carrier mothers may obtain prophylactic treatment.

Persistence of HBsAg, without anti-HBs, with combinations of positivity of anti-HBc, HBeAg, or anti-HBe indicates infectivity and need for investigation for chronic persistent or chronic aggressive hepatitis.

Patients who are negative for HBsAg may still have acute type B viral hepatitis. There is sometimes a “core window” stage when HBsAg has become negative and the patient has not yet developed the antibody (anti-HBs). On such occasions, both tests for anti-HBc are usually positive and anti-HBc, IgM is the only specific marker for the diagnosis of acute infection with hepatitis B. In cases with strong clinical suspicion of viral hepatitis, serologic testing should not be limited to detecting HBsAg, but should include a battery of tests to evaluate different stages of acute and convalescent hepatitis.