All HBsAg screening reactive samples, including pre-donation rapid screening and after-donation reactive testing by ELISA, were sent to the IBT national reference laboratory for donor testing where they were tested for HBsAg, via the ELISA test (MONOLISA TM HBsAg ULTRA, BIO-RAD, California, USA), and anti-HBc antibodies (HBcAb ELISA Kit, Beijing WANTAI Biological Pharmacy Enterprise Co. Ltd., Beijing, China). When the signal to cutoff ratio (S/CO) for the HBsAg test was greater than or equal to 1.0, the ELISA for HBsAg tests were considered as reactive, after which they were confirmed with a neutralization assay (Reagent Kit for the Confirmation of the HBV Surface Antigen, ZHUHAI LIVZON DIAGNOSTICS INC., Zhuhai, China). If there was a positive confirmed result by neutralisation, the sample was confirmed as HBsAg-positive; if a negative neutralisation result was obtained, the samples were considered HBsAg-negative. The reagents used for donor screening tests in IBT are listed in Table 2, and the testing algorithm is listed in Fig. 2 . Table 2Screening Test Kits Used for IBTKit NameCompanySensitivity / Specificity (%)Diagnostic Kit for HBVBIO-RAD (United States)100 / 97.55Surface Antigen (ELISA)HBsAg Neutralization,Livzon (Zhuhai)no dataAntibody to Hepatitis BSurface Antigen (Human)Diagnostic Kit forWANTAI (Beijing)99.4 / 99.1Antibody to HepatitisCore Antigen (ELISA) Fig. 2Testing Algorithm. Note: §: We do not take into account the risk from OBI. HBsAg: Hepatitis B surface Antigen. HBc antibody: Hepatitis B core antibody. ELISA: Enzyme-Linked Immuno Sorbent Assay. IBT: Institute of Blood Transfusion. S/CO: the signal to cutoff ratio