Clinical Significance

The hepatitis B surface antigen, a polypeptide of varying size, is a component of the external envelope of the hepatitis B virus particle (HBV). The blood of persons infected with HBV contains, in addition to intact infectious HBV particles, smaller non-infectious “empty” envelope particles, which are formed in great excess and also contain the hepatitis B surface antigen. The HBsAg determinant a, against which the immune response is mainly directed, is common to all HBsAg particles. Within this a determinant, several HBsAg subtype determinants could be defined as d, y, w1‑w4, r and q. Under selective pressure (caused by antiviral therapy or by the action of the immune system itself), the virus can express many different viable HBsAg mutants (so-called escape mutants). Some mutants might lead to a loss of detection in commercially available HBsAg assays. The Elecsys HBsAg II assay was especially developed in order to detect wild types and a multitude of these mutants. 

The detection of HBsAg in human serum or plasma indicates an infection by the hepatitis B virus. HBsAg is the first immunological marker to appear in acute hepatitis B infection and is generally present some days or weeks before clinical symptoms begin to appear. HBsAg is observed in persons with acute and chronic hepatitis B infections.  

HBsAg tests are part of the diagnostic algorithm used to identify persons infected with HBV and to prevent the transmission of the hepatitis B virus by blood and blood products.  In addition, HBsAg tests are recommended as part of prenatal care, in order to identify neonates at high risk of acquiring HBV during perinatal period. 

_{Source: Roche cobas Elecsys HBsAg test method sheet, V 4.0 2023-07}