Chemi-Luminescence ImmunoAssay (CLIA) for the quantitative determination of Hepatitis B surface Antigen or HBsAg in human plasma and sera. The kit may be used for the screening of blood units, is able to detect HBsAg mutants and find application in the follow-up of HBV-infected patients. The device has been adapted for the use in exclusive combination with the Dia.Pro instrument named SARA.