Principlebioelisa HBsAg 3.0 is a direct immunoenzymatic method of the «sandwich» type in which guinea pig anti-HBsantibodies coated to microplate wells act as the capture antibody and goat anti-HBs antibodies marked withperoxidase serve as conjugate antibodies. The sample to be analysed is incubated in one of the antibody-coatedwells. If the sample contains HBsAg, the antigen will bind to the antibody on the plate. After washing to eliminateany unbound material, goat anti-HBs conjugate to peroxidase is added to the well and allowed to react with theantigen-antibody complex formed in the first incubation. After a second incubation and subsequent washing, anenzyme substrate containing a chromogen is added. The substrate will develop a blue colour if the sample ispositive for HBsAg. The blue colour changes to yellow after blocking the reaction with sulphuric acid. The intensityof the colour is proportional to the amount of HBsAg in the test specimens.