Two commercial quantitative immunoassay platforms are currently available for the detection of hepatitis B surface antigen (HBsAg) within a wide range of concentrations: the Architect HBsAg assay and the Elecsys HBsAg II. However, there are limited data concerning their performance in patients who are positive for both HBsAg and anti-HBs antibodies. In the present study, 127 serum samples from Chinese patients with hepatitis B carrying both HBsAg and anti-HBs antibodies were analyzed. HBsAg levels measured in parallel using the Elecsys HBsAg II and Architect HBsAg assay were compared. There was a significant correlation between HBsAg levels measured by Elecsys HBsAg II and Architect HBsAg assay (correlation coefficient, r = 0.992, P < 0.05). Bland-Altman analysis indicated only minor discordance between the methods (mean difference between Elecsys HBsAg II and Architect HBsAg assay, -0.02 log10 IU/ml; 95 % confidence interval, -0.40-0.37 log10 IU/ml). In conclusion, the results of the quantitative Elecsys HBsAg II were highly correlated with those of the Architect HBsAg assay in patients carrying both HBsAg and anti-HBs antibodies, but differences were observed between the platforms in samples with low HBsAg levels.